It depends how much you want to spend. Decide that, then spend up to your budget. You can get a pair of wheels for £100 (they won't be any better than what you've got), £200 (they should be), £300 (they will be), up to whatever you want to spend.
For club riding and sportive riding, reliability is (to me, at least) more important than super low weight. You don't want to be breaking a spoke half way round the Fred Whitton. Aero rims are always nice, but you won't get decent ones unless you shell out a lot of money for carbon.
I hear good things about Pro Lite Bracciano wheels, which are available for under £300. But I wouldn't rule out handbuilt wheels either, for the usage you're describing. I've had decent, inexpensive wheels made with the rims and hubs of my choice from both Richardsons and The Bicycle Works (in Scarborough, the latter now closed), and from Spa Cycles, and my best ever were some Harry Rowland handbuilt wheels, which I only stopped using after I wore the rims out. With factory built wheels, I've had: broken spokes, uneven spoke tension, out of true wheels, out of dish wheels. With handbuilt wheels, I think I've broken one spoke, ever.
Don't take that as gospel. There are poor handbuilt wheels. There are great factory wheelsets. But if I were in your position looking for new wheels for club riding, I'd probably call Paul Hewitt (Google him). In fact, I've got a pair of his on the way to test (along with 15 other pairs, all below £300), so ask me again in two or three months!
Don't scrimp on your tyres either. Good tyres are one of the best value upgrades you can make. Michelin Pro 3 Race are very nice, although the tread cuts up a bit in winter, so they're better for a Sunday best bike or summer usage. I have Michelin Krylion Carbons on my training bike. Other good options include: Continental Grand Prix 4 Season (for all year use), Schwalbe Durano (ditto) and Vredestein Fortezza Quattro. They're all a decent balance between good rolling performance, grip and puncture resistance (though you can get tyres that do any one of those things better).
One word of warning on Mavic: I know one pro wheelbuilder who will no longer use Mavic rims, due to problems with them cracking. I wouldn't say don't use them (I've ridden some very nice Mavic wheels), but I would recommend checking the warranty regarding replacement, so that if you are unlucky enough to get a problem you won't be wheel-less.