Buying online is somewhat pot luck - how do you know what size fits correctly, and what shape of boot relative to how wide your feet are, narrow or wide your ankles are etc? Snug fitting boots are much more important in skating, where the power and control is transmitted through a sensitive blade to a responsive surface like ice, than in more or less any other sport - and much more important and less forgiving than for ordinary street shoes.
You should be able to send boots back if bought online and they don't fit, but some items are deemed "special order" and either can't be refunded or are subject to a hefty "restocking" fee.
I know there's a rink in Swindon, and now one in Bristol, so there may be shops there. Otherwise some people have said that Everglides on the south coast are good. There are rinks in Birmingham and Cardiff too.
I suppose you could try on as much as you can in your local shop and possibly by process of elimination (trying on sizes too small and too large, if they have them) find the right size, but I would travel further afield to avoid disappointment. I went from London to Eastbourne to buy my rollerblades and have never regretted the time and money spent in finding the right boots.
Good luck.