Most of the medium to large festivals have a variety of workshops including working on various aspects of singing. I know that Chippenham usually has a number of voice workshops but I don't know much about the festivals in the north west. You might try using the EFDSS site to check out what each of the different festvals near you offer. Their search page might be a good place to start. The url is:

www.rootsource.co.uk
hope that's of some help. Jude