A practical procedure for legislation -

I would suggest:

A rolling legislative programme.

1. Firstly, an act to provide that in respect of any person’s total landholding in the UK (or any body or trust s/he owns or has effective control of) which has a current land value of more than £10 million and cannot be proven within 3 months of the act’s royal assent:
(a) to have been bought for fair value in the last 160 years; or
(b) to have been inherited (either immediately or though successive prior inheritances) from someone who has bought the property for fair value in the last 160 years,
that land will become immediately registerable and must be registered by (and at the cost of) the land owner and valued within 6 months.
(Much of this land remains unregistered and is, frankly, used to hide land ownership.)

2. The Crown Estates Office would become the body overseeing the whole process.

3. If the land you own is worth less than £10 million or (even if you inherited it) at some point in the last 160 years was purchased in a normal property transaction for money then your land is entirely exempt. That means almost everyone in Britain.

4. Specifically, though, the land holdings of any Duke, Duchess, Earl, Viscount, Marquess, Baron, Baroness, Knight, Baronet, Peer or Peeress or the land held by any body or trust over which s/he or his/her family has legal or effective control must be registered, identified and valued. Where the Crown Estates Office has reason to believe that any other person holds land in the same position as any of the above they should similarly become registerable.

5. The Crown Estates themselves, the royal Duchies (e.g. Lancaster and Cornwall) and lands owned/in the control of the monarch personally (as Mrs. Windsor) and by reason of her position as Queen should be similarly treated for completeness sake.

6. A caution should be entered on the land register, register of land charges and local land charges register to the effect that this land is subject to possible vesting in the Crown in Parliament and the proceeds of the sale and or land itself (if not sold for fair value) could be recovered by the Crown Estates Office. The purchaser for value would be able to sue the aristocrat for losses should the land or proceeds subsequently be recovered by the Crown Estates office. The valuation would not take into account the obvious drop in land value which would occur by reason of the caution/entry.

7. The caution would be removed if the Crown Estates Office determines that the value of the successively inherited land holding in its entirety is less than, say, £30 million. Above that, the landholding will be designated by the Crown Estates office as ‘crownable land’.

8. Any proposed or attempted sale or transfer of any part of such crownable land would need to be reported to the Crown Estates office (and specifically by any solicitor/licenced conveyancer involved). The Office shall determine whether or not the proposed sale is for fair value on the open market and no completion/registration of the sale could take place unless a certificate attesting to a fair value sale was delivered to the land registry by the Crown Estates Office . The sale proceeds would be held by the seller’s solicitor in escrow pending further determination by parliament or the landowner as to those proceeds.

9. Once this crownable land is registered, identified and the value confirmed, then the owner should be offered an amnesty period of 12 months within which to elect to return the land to the crown voluntarily and gradually within 4 years. Upon which voluntary transfer, any single residential dwelling in which s/he actually lives upon in the estate (or estates) may be retained and properly transferred back to them by the Crown Estates Office along with a designated appropriate amount of attached land. This shall form due compensation under the European Convention of Human Rights.

10. After the amnesty period the land identified by the Crown Estates office as crownable and being held through landed aristocratic heredity alone and which has not been subject to voluntary return, should be returned immediately and in its in its entirety to the Crown in Parliament (possibly as a forfeiture of the upper feudal lord – the crown) through its vesting in the ownership of a committee of both houses of parliament This shall be known as ‘crowning’. Its management and sale to the private sector shall be managed by the Crown Estates Office.

11. Any attempt to avoid registration or to alienate or sell the land without the consent of the Crown Estates Office or destroy the value of the land would be an imprisonable criminal offence. The land would immediately vest in the Crown in Parliament as part of the criminal assets confiscation procedures already in operation elsewhere in the criminal justice system.

12. Alternatively, and to avoid ECHR complications (where the court is loathe to intervene where the method of intervention is taxation) the land shall become subject to an immediate land value tax of 40% of its value annually and 80% of its income annually until such time as the owner voluntarily returns the land to the tenant in chief – the crown. Or until death where the property inherited will be subject to a land inheritance tax of 100% of the value above the standard £325,000 IHT threshold (non land assets included before land assets for this purpose). The Crown could also even refuse to consent to the investiture of the land in the subsequent heir, thus causing the land to fall back to the Crown under feudal principles.

You could also take the same approach to land owned by Oxford and Cambidge Colleges and even the Church of England.

Feudal principles could quite legitimately be brought into operation as we haven't formally abolished feudalism yet in England: escheat, forfeiture, relief, heriot, wardship, all that kind of stuff. It would be interesting to see the ECHR looking at this.

Magna Carta might be a problem - because while we pretend that it was a great step forward for liberty, in my view it mainly just enshrined the property and political rights of the barons as against the crown - established and enshrined the aristocracy that is the problem in the first place. There's other good stuff in it, I'll acknowledge!

As most of the Aristocracy started off as military tenants under feudalism, we could insist the aristocrat is ordered into a war zone on behalf of the UK, since refusal to do so would enable forfeiture of the lord's lands. Taking up arms on behalf of the monarch is the basis of the feudal system and refusal to do so means the land falls back to the crown! The old principle was that the king was "every man's heir".

The legals are easy. The political will is harder!