I stayed at the Damson Den B&B near Windemere a few years back and it was lovely. I've just checked their website, and they do weddings from around £55pp for the whole day.
It was a stunning venue - rooms were nice and clean, and the dining room had a beautiful view over the Lake District.
They also have a swimming pool and onsite spa. I had a massage, and the woman was great - perfect for pre-wedding day prep? ;)
The food was really tasty as well.
They have a wedding brochure you can download.
Disclaimer - do bear in mind this was a few years ago now, but if it's still as good as it was then, it would be a lovely place for a wedding!