For proper work experience, rather than just a visit, there is usually a couple of years waiting list at active RAF bases, you can't get it sorted in the next few weeks. They also usually only take people via a specific school scheme (Jupiter, Juno?). Far better, try your local flying club, see if they have got anything they can do for you.
The problem with aviation based work experience is that you aren't qualified to do anything, so you can't actually do much work 'experience' you can just sit and watch and ask questions. There's nothing much at all you can do if you want to be a pilot - not worth bothering imo. But if you want to be an engineer, or work in ATC then ask specifically if you can see those departments.