Coke611,
You're right about Stapleford not being ATC it's an air/ground service which is rather different. Unlike ATC, air/ground provides pilots with information only and not instructions. It might be useful to understand the differences for an interview but unless you know what ATC is it would probably be confusing to visit an air/ground service at this stage (and it won't be representative of the working conditions at an ATC unit -
was that tactful enough).
You might be better off trying Luton or Stansted. I'm sure both would be pleased to help someone keen on a career in ATC. The drawback is that you'll only see aerodrome control at those airports and that's only a small part of the job - but it's a good start.
And good luck.