I did the Day Skipper theory last winter as a semi-intensive (three alternate weekends) and then the Day Skipper practical (four days afloat) in March. The RYA web site or NFSS (National Federation of Sea Schools) are good starting places for looking for centres. I did my theory at Bisham Abbey Sailing School in Marlow, Bucks, a good place but not much good to you if you're the other side of Bristol.
Have a look at:
For the Coatstal Skipper with commercial endorsement you're supposed to have 30 days at sea, 2 days as a skipper, 800 miles logged and 12 night hours. But here's the catch. The RYA/DOT definition of a day at sea is a 24hour period afloat, with time ashore for shopping, pub, etc, but spending the night on the vessel. I don't know if you're allowed to sneak off to a hotel or camp site whilst the boat stays in a marina!
Good luck with it,
Just had a thought - lots of the yotties fly off to the med to do their courses over the winter, and some of the sailing schools over there also do motor cruising courses. No reduction in quality by doing it at a sailing school either, they often have a dedicated motor cruiser trainer, and even if they dont, the yotties are just as good a laugh as ribsters. NFSS again for this. I got lots of info about this at Earls Court last January, so I can hunt arounnd for it if you want.