The UK government can compel you to give them whatever they want by indirectly acting via the US Govt. They will pressure the US Govt the US Govt will act on you. So even though on paper they can't directly act against you, rest assured, they can. Lets pretend that the US government would not comply with requests to compel you to give them something, you can still be sued or otherwise acted upon in such a way that you would have to expend time / resources to fend off the request / lawsuit or whatever. So to say that the UK Govt can't do anything to you because you don't have any assets there or don't want to go there is kind of silly IMO.