+1 for either
Decathlon or for a good used bike.
Halfords can be hit and miss in terms of service (I'm lucky, two out of my three local branches are good), but their bikes are actually pretty decent once you rule out the bargain basement BSO.
A really cheap bike is more trouble than it's worth. Others have already mentioned the componentry made of cheese, but cheap bikes are heavy. Really heavy. Combine the two and it makes cycling a chore. Which kind of defeats the point.
Most important thing is to find a bike, that as well as doing what you want it to do, puts a smile on your face when you ride it.