Sounds like you've been unlucky with the weather and with the area itself, it sounds like it's too quiet.
Our teens quite like wandering around small villages and towns as long as there's a good selection of shops to explore, places to have lunch, places to eat ice cream.
We play board games if it's rainy, or do quizzes, but then we do those at home too and they enjoy them there so we're in the groove with that already.
We hire bikes, go on bike rides. We go and nosy around at whatever the local area has to offer, castles, visitor attractions, zoos, national trust places etc. In the past we've found outdoor swimming pools, crazy golf, cinemas.
We love eating out so go somewhere with good restaurants.
We've had so many fantastic UK holidays, but mostly we've been incredibly lucky with the weather.
Try not to let this put you off ALL UK holidays, just pick a better area next time, with more to do locally.
For our upcoming UK holiday we'll have beach nearby, lots of little towns to explore, and the river for paddleboarding and canoeing, plus we'll hire bikes and eat out as much as we can afford. Can't wait.