Just because it has to be from Halfords doesn’t mean it has to be something offered on their shop floor. They can get in pretty any main stream bike you fancy. Only problem is it’ll be at RRP. They can’t get things like Focus, Canyon, Planet X, Ribble, etc that are sold direct from the company or a sole distributor.
I got them to order a Giant in a few years back, don’t think they were selling Boardmans then though. Definitely worth a look around to see it there’s anything else you like, that said I think my search would start and end with a Boardman too at that price range.