Here's my list from a previous thread. If you want the links for any of them, go to the original thread which is here :

Folk Southwest Easter School
Folk South West
Not yet on FSW website, but dates according to John Kirkpatrick's site are 8-11 April this year. This offers probably the best, most trad/folk-focused singing masterclass workshop. Fantastic tutors (eg Chris Coe, Tim van Eyken, Eddie Upton, Shirley Collins).

Folkworks
Folkworks Summer School
(then click on Adult Enrolment for 2009 programme)
Used to have a Solo Singer masterclass, but I think their voice class is now more group/harmony singing. Check when the 2010 programme is put out.

Maddy Prior's voice courses
Maddy Prior
Run by Maddy, usually with Abbie Lathe, and sometimes her daughter Rose Kemp teaches too. All singing/voice classes, with at least one masterclass workshop.

Baring-Gould Song School
Baring-Gould Song School
Past tutors have included: Sandra Kerr, Martin Graebe, Sian Graebe. Participants create their own programme, but masterclass/solo singing is always included

Farncombe Community College
Farncombe
Sandra Kerr used to teach voice here, but doesn't seem to be on the programme this year - but Jo Sercombe's Solo Singing looks good, and Frankie Armstrong teaches here too.

Lewes Saturday Folk Club workshops
Lewes
Class with Shirley Collins coming up! check with them about workshop content.

Counterparts (Helen Porter)
Counterparts
Not folk, more jazz/classical oriented - but Helen's actual voice technique tuition is first-class, in a master-class format.

The Tuscany Project
Tuscany Project
Again, not folk, but more musical theatre/cabaret/classical. This week-long workshop is pricey, and takes place in Italy, so more of a special summer holiday. Course is intensive, challenging, and with superb voice work tuition.