The TV licensing people have full access to your house if they inform you of their intent to check up on you twenty-four hours in advance. They don't, contrary to popular belief, have to inform you beforehand.
TV licences are specific to a 'living area'. In halls of residence, for example, in a flat with six bedrooms and a central living area, if you can lock your doors, then each room counts as a separate area. If you have a TV in the main room and one in your room, that's two licences - a potential of seven for the whole flat (or £840 all told), which is an absolute scandal.
We had this situation last year, and three of us had licences while the other three did not. We got a letter warning us to get licences for those addresses that were not licenced, but there is no way that they can prove in that situation that you are watching without one.
The TV detector van is in no way accurate. If you have two TVs in your house, one upstairs and one downstairs, it might be able to differentiate between the two, but that's when the units are far apart from each other. In a hall, with rooms placed tightly together and with a TV in each one, you probably won't get caught.