I'd back up the multiple visits thing. I managed to get to Swanwick, Heathrow Tower & Lands End & all of them were beneficial and helpful. It's not just the visit but the whole gang of ATCO's you can ask for info that really helps too.
As others have said don't overlook the regionals as they're often able to give you more time if it's a quiet spell. Definitely helps to do En-route and airfield as the ATCO who runs your tech interview is likely to specialise in one of these. Although the interview is certain to cover aspects of both it can't hurt to be able to refer to visits for questions you're asked.
Best of luck to you.