You could simply get a friend to play the part of ATC for you - CAP413 and a bit of coaching from you will give them everything they need, especially if you only want to practice circuit calls for now.
Or, if you want something that will keep you on your toes, you could try Vatsim - the virtual-ATC network for flight simmers.
You can talk to real people acting as ATC and learn in a much more forgiving environment - though most people take it just as seriously as the real thing. It is very realistic.
You probably won't be too welcome if you try to do circuits at Heathrow, but at any of the regional airports the controllers are often glad of the extra work trying to squeeze you in between the airliner traffic. It's fun for them and for you.
As I type, there are no special events on but Edinburgh, East Mids, Farnborough and Manchester Towers are all manned - I'm sure you would be welcome to try circuits at any of them.
Vatsim works with Flight Simulator and X-Plane.
VATSIM United Kingdom Division