If you are looking for a family vacation in the UK then you may wish to consider booking through a company such as Pontins or Butlins. These companies have been around for years and specialise in offering fun family holidays at a number of locations all over the UK.
They tend to cost a little more than a standard campsite, but with the wealth of entertainment on site you can get away with spending very little when you are there. They offer a choice of accommodation, which is self-catering, meaning that you can also take your own food with you, so do not have to fork out on dining out every night / day.
There is a wide choice of beach and inland accommodation all of which vary in price dependent upon the time of year you go.
Some people say these places are a bit cheap and cheerful but that might be exactly what you are looking for.