You'll be
expected to visit ATC facilities prior to your second interview, but there's nothing to stop you getting ahead and trying to go now.
Since you're in London some of the LHR and LATCC people will certainly be able to help organise this for you.
Choice of posting, now there's a laugh! You can request but it will by no means be certain that you'll go where you want
initially.
It may all change by the time you get in (thinking positively here) but everyone will do an Aerodrome course after their Foundation courses and after their Aerodrome OJT (on the job training) you will be streamed to either airport or centre work. As you can see its a long way off yet.
Read everything you can about ATC and make the effort to visit lots of different units both large and small. You've made a good start by finding PPRuNE! It will serve you well!
Good luck
Edited to correct spelling
[ 15 November 2001: Message edited by: cossack ]