Butlins etc isn't really my thing either, but I think I may have struck middle ground between the kids needs and mine and have just booked a caravan holiday on a park with indoor swimming pool, adventure playground, 200m from the beach, but minimal "entertainment". I am thinking that my dc may be bored and clingy, especially if the weather is bad, if there are not other children to run around with and make friends with, and bored and clingy children will not a relaxing break for Mum make!
I think a b and b might be hard work if the weather is bad, as you will have to stay out all day in the rain if it does rain, or is cold. Also, you will probably be sat in a darkened room all evening being quiet as a mouse once the children are asleep. With a caravan at least you have a base you can use all day, with separate rooms, so you can relax in the evenings once kids are in bed.
I went to stay in a Youth Hostel with the kids at Easter, and asked for a room next to the lounge. This meant that once they were asleep at night I could take a baby monitor into the lounge and have a glass of wine and read the paper.
It is me again, but have you checked out the website www.singlewithkids.com? They do group holidays for single parents. I haven't used them yet but was really tempted to book one of their holidays this year, but the only one I could afford was a week we can't do. I know other people on this board have used them and have heard good things.