I'm surprised no-one has mentioned the various "Song and Ale" events, in the UK at least.

If you wanted just singing you could go to Verwood for the "Wail", a weekend of singing with a mediaeval midnight feast thrown in. Or you could try the Pennymoor Song and Ale which has tunes as well as singing, again over a weekend. Both these have a minimal ticket price which just covers the camping (and the feast at the Wail).

I presume the Fox and Hounds weekend (Music and song) will be running again this year at Bridestowe (charity collection only but you would have to pay the pub for your camping/bunkhouse/B&B). If you're more of an audience than a participant, there's the Dulverton Folk Festival (even the concerts are free).

I do know of others, such as the Whittlebury Song and Ale, but I haven't been to that for years so I can't say much about its current set up but I know the people who go to it are very complimentary.

Sorry that these are down south but I'm not much of a festival goer myself and only tend to get my information by word of mouth so I'm limited in my sources. My experience is that, once you find one good session weekend, you'll hear about the others from those that are present – same as finding good sessions in your local area, the best ones are often the least advertised.