He also doesn't claim that it's strong evidence on its own. That uncertainty is actually part of what the talk is about. The evidence must be taken in combination with other issues, e.g. his claim that the Brits do not even try to hide their surveillance (and therefore have been photographed) and the sophisticated surveillance device he found in his crypto phone.
Counter surveillance training isn't hugely difficult but it takes a certain amount of skill. The idea that the Security Service would want to be seen is a little strange. Their operators are excellent and they would be very very reluctant to expose themselves easily to send a message or bad tradecraft. If they want someone to be clearly seen they send a copper.
There are many other possible options. Basic trained police (some have been exposed before in Wikileaks related stuff) or a military team, foreign actor or quite possibly a corporate intelligence team. Perhaps even criminal elements.
Working to help manage security for NGOs in London/elsewhere often means my organisation works essentially a counter-intelligence role against corporate intelligence (many of whom were often working on behalf of states). They are very prominent in digital and physical surveillance. Many times we identified and exposed very big operations targeted against activists - harrassment, hacking, physical surveillance, bugging etc. It's was one of the reason we built our open source Umbrella App (https://www.secfirst.org) to make it easier for activists and journalists to be able to learn about and manage things like surveillance. Covert method of entry teams in the UK are known as exceptional (you'd have to be working in Northern Ireland for decades), there's pretty much no way they would get stuck by a commercially available key. Also it's highly highly unlikely they would try make an entrance when the person is so near to their home. Especially if it resulted in damage. For such a sophisticated door they would have planned such a thing well in advance. It's plausible of course, maybe they were just lazy or whatever, but there are a number of possible other reasons.
I guess, but if they needed a ruse to delay someone at immigration it'd be much simpler to just say that the computer was slow or something. No playacting required.
