Great to hear your insight regarding the US recruitment process James(!).I always did - maybe this is a US thing but all my friends who did consulting recruitment in the US were told that interviewers looked to see if they received a thank you e-mail and that it could make the difference between two very equal candidates. That said, I do get the sense that it is not such an important thing in London TC/VS recruitment.
Regardless, I always believed that it was a polite gesture to send a kind note thanking the interviewer for their time. I also do this whenever anyone has a call with me or helps me out with something - it is good practice throughout one's career to recognise when others have invested their time on your behalf and thank them for it. I often received some really lovely responses (some even telling me how well they thought my interview went which I feel like they were not supposed to reveal)!
I never even considered that interviewers might incorporate this into their decision-making process. That being said, I think you are right about emails and UK firms.