Since a fair slice of the British army is based around Salisbury, and the CNO lived nearby it's not that much of a coincidence. If you'd ever been there, you would know that.
For this to make sense, am I to assume, according to this twatter consipracy theory expert you quote, that 2 Russian GRU officers being there were merely 'visiting Salisbury to admire cathedral spire' and had nothing to do with what happened while they were there?
Occams's razor and comon sense: Putin getting rid of someone he admitted he despised rather pubicly makes more sense than any nonsensical assertion it was a UK Gov't conspiracy to smear Russia. Litvinenko's poisoning in London with Polonium was a not dissimalar MO. One must ask
cui bono, surely?
The UK Govt is not pure as the driven, I would never seek to hold it up as anywhere near a paragon of virtue. However, this stuff has about as much credibility as the assertion the moon landings were faked.