Forest of Dean too far for you?
We've done the FOD a number of times over the years with friends and family, and now my friends are trying to persuade me to try the mountain bike trails rather than the family trail there.
I see you've written Cirencester for your location, I remember doing a cycle path in Stroud with my children quite a few years ago, then further afield there's loads around Bristol from the well known Bristol and Bath cycle path, to routes around the docks, the path to Pill, and Ashton Court for example.
And if heading South from you there are the paths along side the River Avon that I've done several sections of.
Cirencester itself is one area I don't know I'm afraid, if you were wanting more local.