Most Units in the UK are very welcoming in having visits for those coming through the selection process. You're better off choosing a Unit which has a Tower landslide to avoid having to go through the security process.
In reference to the above and knowing what the AIP is about, don't worry, the final interview is purely competency based and has no technical questions (unless it has changed since I went through it in 2010). It's probably worth having a quick online but nothing more.
What you will probably get asked is whether or not you have gone for a visit (so 100% organise one) and they'll ask you a few questions about what you saw and understood so don't be afraid to ask lots of questions when you're upstairs when they're not busy!
