Try "www.boatlaunch.co.uk" (and remember to send them an account of any sites you visit so they can update the description) . It lists a number of good sites around the coast. I would also agree with the recommendation for "Where to launch around the coast".
Whatever the web site or book says I would always recommend viewing any launch site yourself (especially at low water) to see any hazards which are not mentioned.
Be aware that a lot depends upon the type of boat you have. I trailer sailed for a few years with a 19' long keeled sailing boat and found that many of the slips listed in the various publications were just not suitable: OK for a speedboat or the like but not for a boat with 500mm draught at the stern, which needs a good steep slip. We got caught out a number of times, which made it all too stressful.
Ive had the same experience as Bargee. Trailer sailing seemed the way to add a bit of variety to South Coast Sailing without extended passage making.
OK so I wouldnt (under current terms of employment!) have been able to get as far as Falmouth, and up the East Coast, but the lack of information, together with the fact that most of the slipways listed had major drawbacks making the launching of a fairly heavy boat so traumatic that Mrs O'H has finally put her foot down and refused to participate!
We found that local knowledge with careful on site research was needed BEFORE bringing the boat along: this was a lovely excuse for several weekends away in the car trying to find a slip that met our requirements (not too steep, good surface, manageable access, long term parking within reach at a manageable cost etc etc), but altogether not the way we wanted to begin and end our holidays!
And car parking? Surprisingly few slips have anywhere you can safely leave the car and trailer within reach while afloat, and those that do tend to charge a small fortune, running in some cases in to 3 figures for the weeks parking!
So what sort of problems did we meet? Road access was sometimes very limited - narrow steep lanes, crowded access making reverse manoevering dangerous, slippery slips (mud, weed gravel etc), slips that stop or drop suddenly in to soft mud, boats/cars gear etc stored on slips making accessbility problems, strong cross tides over the slip making placing the boat on the trailer well nigh impossible, excesss wash from passing boats making ours crash down onto the trailer damaging both it and the hull, slips more exposed to open water (one, decribed as 'sheletered' had a 2 foot surf running on it when we arrived!).
But with careful research and plenty of time to wait for the right conditions, trailer sailing is a very good way of finding new waters. I would say nowadays, it is NOT for those who are limited to a week or 10 days leave at a time.
Oh the joys of creating a 3 mile tailback on the M11 as we stuck to our safe 50mph! And the unexpected bonus that the road ahead is always clear when you are towing - no more looking down the exhaust pipe of the car in front!