Chippenham Wheelers hold sessions at Castle Combe on a Wednesday where you just bomb around the circuit. Open to non-club members.
There is a sportive where you cycle from Brsitol to W-S-M and then get a coach back again. I think it's in July but can't be certain.
There is also the Great Western Sportive that sets off from Castle Combe and goes over one of the Severn Crossings.
Best to check out cyclosport.org or Evans Ride It. BHF also do bike rides as do Bike Events Online. (Sorry, can't do links)