And Canadians :-) . . .

Pre Clearance is handy; and the CBP officer I spoke to for my Global Entry/Nexus interview told me he lived about 5KM from me (in Canada); they get a 2 or 3 year visa (can't remember which) and he had bought a place to live while here. So I guess some are temporary and some are permanent based on other posts.

However, it requires a change to immigration law to grant them permission and visas etc. The US officers can detain then call local police in Canada IIRC to arrest you if they're looking for you, and then go ahead with extradition, but on the other hand as a previous poster pointed out they can also simply board the plane on arrival if they like.

I think the value of putting pre-clearance and LHR would be spreading out the processing load to various locations. I echo another poster's comments that I have had some horrendous lines in LAX, some quite bad ones at EWR, and I will not go to MIA because of some of the stuff I hear about it (my family does not yet have global entry and I usually travel with them; by myself I wouldn't care).

It's doable if they want to, but there may be other reasons (that aren't of a practical nature) why they (either BAA, UK gov't, or US gov't) don't want to.