Leisure Lakes have improved massively over the years, used to be full of sneery staff that wouldn’t look twice at you unless you’d got several grand burning a hole in your back pocket, but much better these days. Also got 2 very good mechanics (not that the old one wasn’t good in car he’s reading! 😉 ).
Cheltenham cycles are a very friendly bunch of guys. Pete the mechanic posts on here occasionally, he’s very helpful and knows his stuff for sure. They may not have as big a range as LL but what they do sell is all good quality and they know what they’re talking about.
Williams is a bit of an oddball, you can go in there and find an absolute gem on occasion, usually a NOS part from about 1998, but in general though the staff are very friendly and knowledgeable it is more geared at commuters and cycle tourists than MTBers and roadies, which probably works quite well as the other 2 shops have this area covered well.
Further afield, as said above there’s Noah’s Ark in Stroud. Used to be the de facto bike shop in Gloucestershire (and kind of still is for snowboarding) but if I’m honest last time I went I found the guy who owns it too abrupt and not very helpful which put me off a bit. It’s also a long way from where I live now. There used to be a small branch of it in Cheltenham years ago but sadly no longer, as this was a pretty cool little shop.