All good advice from everyone. The only thing I would add is that if you can afford to get yourself on a few training courses (usually only a week long, or less) it will go a long way toward making you stand out as a more valuable volunteer (and therefore someone The Wildlife Trust or RSPB or similar, might offer some contract work to or invest some time in helping you get more experience). These qualifications could also lead to getting work with other counrtyside contractors doing work such as hedglelaying, tree surgery, scrub clearing, fencing etc. It's a good way to get some experience, money and time spent on the job. The courses/certificates I'm talking about would be your NPTC chainsaw ticket, knapsack sprayer certificate and brushcutter certificate. All should be availabe from a local agricultural college. A good investment that can get you earning some money fairly quickly if you're really serious about doing this kind of work and get a few good contacts.
Go for it! I did!
Good luck.
Cheers,
Jamie