Given the discussion about Concorde (and the fact that it practically had two routes), it's probably worthwhile to point out that the UK has long had a very strong distinction between landside and airside, and airport security didn't change anywhere near as much here as it did in the US after 9/11: the risk of terrorist attacks on aircraft had long been around in the UK. The big change was not being allowed sharps in the cabin, from memory.

Security didn't deter from the LHR–JFK route before with security at the UK end, and I doubt it'd deter from it at the US end.