Hi Peter
I am just completing a MAppSc in Marine Biology and Natural Resource Managemnet at James Cook Uiversity in Townsville Queensland. The course is full time for 18 months and I would highly recommend it as it provides lots of options to specialise.
I also work for a not for profit organisation: ReefCheck Australia Thought I'd chime in as this or a similar organisation might give you a few other avenues that you could consider rather than full time study. Reef Check is a coral reef monitoring organisation that trains community volunteer divers (minimum qual: PADI rescue and 50+ dives) to collect data on reef health. We run video, substrate, invertebrate and fish surveys as part of our monitoring programs in Australia. The data is being intergrated into an online spational database for use by a wide variety of marine science professionals and other interested parties.
The training programs give a good practical insight into marine organism identification in coral reef systems. We also monitor impacts such as coral bleaching and reef damage. The courses are inherently very practical and designed to train any enthusiastic competent diver to understand reef ecology processes and use these skills to participate in coral conservation research.
Obviously Aussie is a long way from the UK! But the Reef Check network covers 80 countries around the world ReefCheck and volunteers who have completed the training program can contact other countries in the network with the potential of participating in their monitoring programs. We run a number of courses per year that are full time running over 2-3 weeks depending on the weather!
So as not to seem biased it is worth noting that there are a number of other organisations with reef conservation goals such as Coral Cay. Coral Cay is based in hte UK and hence might suit you better.
Obviously these are both focused upon coral reefs which are only one (albeit very important) of the ecosystems that are covered in a marine biology degree. It is worth noting that the subject is very broad and hence as many other participants in this thread have mentioned getting out there in the water is the best way of working out whether you want to give up your day job! Personally after 10 years working for a corporate I can highly recommend it! 
It is also worth noting that you can become a volunteer at several of the marine research stations on the GBR: Heron Island, Lizard Island, Orphues Island etc
I hope this provides a little more seafood for thought. 