They've done it in countries to which they are way less "aligned" than the UK [0] and the UK has historically been the US's strongest partner on all things "national security/terrorism".

In that context, I wouldn't really be surprised if it's happened before, tho the blowback potential, if it should become public, would be massive so it will probably be kept under extremely tight wraps.

[0] https://www.opensocietyfoundations.org/voices/20-extraordina...