(from the mask thread, fits here better)
You really think that T&T go to the trouble of figuring out who was a close contact or not, based on all the stories that have come out about it? I wouldn't be surprised if they just went through the list provided by an establishment of who was there at the same time and told them that they might be infected. Hell, that is all that they can do, unless the restaurant starts noting down table numbers, seating positions and handing over CCTV recordings
If they're doing it properly (and the local authority teams will be) they will contact each person and ask them some questions to determine risk (e.g. "do you remember where you were sitting? did you socialise with another table?), not just tell the whole place to self-isolate.
Some businesses
are collecting table numbers and these are the best ones as they make the job easier! The visible list at the door (=GDPR breach) does show organisations not taking it seriously.
If you
really do contact tracing properly yes, you do go into CCTV etc - doing a good job is quite invasive (compare it to, for example, the way high level security clearance like DV is done). For unusual serious one-offs I'd imagine they do do that, e.g. if they find a case of Ebola in the UK (I know the spread of that isn't the same).