US Constitutional protections and British law
I do not see why HM spooks aren't treated the same in the US as the home grown variety, and vice versa in the UK, with regard to this issue. These are sure to be joint operations in either country. Apply to the courts for a warrant in the US, apply to the Home Secretary in the UK. Follow the respective laws and procedures of the country in which you want to conduct your operation.
It certainly does not take months for US spooks to get the court order they request, nor I'd guess, months for HM spooks to get approval from the Home Secretary. Do what needs to be done to make this reciprocal.
Any visitors to the US should, without question, be afforded the same constitutional rights and protections as those given to US citizens. No circumvention of these rights, by anyone, should ever be considered, let alone granted, to anyone, whether foreign or domestic.
I am given to understand that if I visit the UK, I will be subject to British law, as well as those applicable EU laws, while enjoying the same rights and benefits as do citizens of the UK and/or the EU. If this is true, and I have no reason to believe otherwise, then that is as it should be everywhere, especially in a country (the US) which purports to be a beacon of rights and freedoms for the rest of the world.
That's enough for now, I'm off to read The Fiver, have my dinner, a wee dram (or three), and a good night's sleep. The only laws I'll pay the slightest heed to this weekend are the Laws of Football. May all your sides be Victorious!