As Steve says, it's worth studying the course book "RYA VHF Radio (inc. GMDSS) (E-G22)" beforehand and, if you want to "cheat" the "RYA VHF Radio SRC Syllabus and Sample Questions (E-G26)". Both are available as ebooks. The course I did was not as many hours as Steve quotes and I hadn't heard they were going to a two day course.
My understanding is that there is only the one RYA course which assumes no knowledge but, if you already have a non-DSC licence most centres will give you a discount. My experience was that, if you already have USED a VHF radio, the course is fairly easy. Problem is, although people might have a VHF radio, most have rarely needed to USE it!
National Coastwatch volunteers (at least at Calshot) get to take a short range radio training course for free. Having done the course they still need practice and many Watch-keepers, if out on the water, call up their station for a radio-check simply for practice in using the radio (both for those on the boat and those at the station). You can call up a local Coastwatch station direct on channel 65 and ask for a radio check, the present weather, or, if you have missed it, the inshore weather forecast. It gives both the Watch-keepers and you practice with the radio.
Unlike the Coastguard, Coastwatch stations are not inundated with calls for radio checks so feel free to call them on channel 65. The call sign to use is "station-name NCI" e.g. "Calshot NCI". While most stations are open every day for most of the daylight hours, some are not. You can find out station opening times on the web at...
https://nci.org.uk/content/station-map-and-list (click on the station name for their details). You can also phone stations which can be useful if it turns out you do have a problem with your radio. Recently one person phoned Calshot while trying to call us on his radio - we could tell him that we were picking up a carrier when he transmitted but no voice, which suggested a microphone problem.
A couple of tips: make sure your radio is set to "International" channels, not "USA" or "Canada", otherwise you won't hear a reply from a Marina or Coastwatch, nor will you hear the new Coast Guard safety information broadcasts when they switch to new channels in the autumn. Also: make sure you can spell your boat name and call sign phonetically even when in a panic - having a sticker by the radio with name, call sign, and MMSI number helps!
Peter