Trump would probably qualify for uk residency on the basis of the value of his business. (Millionaire can move here if they do enough business).
If he goes bankrupt that might be an issue...
...however he probably could earn enough through book deals etc etc.
So i think that would probably be his most likely route to a uk visa rather than via his ancestry.
Having said that the security issues are problematic. He might expect the British government to provide this or some how fund it. (Not happening. Politically toxic but British government). Bringing US State security to the uk would require permission of the British government (and they might not want that because they would him and Garage (sic) agitating which isnt in the interests of them and the state in general) and the US government (who may not want to foot the additional cost and the diplomatic bullshit it produces). Remember the UK government will also be keen to please the new administration so if he does come here with the intention of staying it will have to be with the approval of Biden (theres an argument Biden might just what shot of him winding up crowds and the hassle of a court room drama though).
Where it gets interesting is if Trump were to come before 20th January because i believe if he wanted residency he legally has to apply to the home office before entry. I can see him kicking up a fuss and expecting special treatment but again see about about Biden's approval.
He certainly wouldn't be allowed to legal to supply his own private security on his own terms (armed with guns) in the uk without the permission of the government and thats not going to wash when he could have US state security.
So yes Trump could come here, but he's going to have hoops to jump through and may have to abide by certain conditions he might not like and he will even as a former president have to ask permission on some level to do so from the new boss which he sure as hell isnt going to like!!!!