The VHF course is run over one day and involves theory and practical sessions. There is a small multiple choice exam at the end of the day.
The course costs from around £70 to £100 plus £20 to the RYA for the certificate.
You can find out more info on the RYA website and search for training centres that run the course. Try and find a centre that uses real radios rather than PC based simulators. I would recommend Powerboat Training UK based at Cobbs Quay, Poole. http://www.powerboat-training-uk.co.uk