The cheapest way of getting a bell tent is to buy it from Obelink in the Netherlands. Unfortunately, due to a restrictive agreement with suppliers over here, they do not ship bell tents to the UK. So to get one, you would either need to take a trip to the Netherlands, or find somewhere in mainland Europe they could ship to and collect it from there (I had them ship an inner tent to a campsite in France which worked well, but you need to agree it with the site first to make sure they will accept delivery).
If that all sounds like too much trouble, also check Canvascamp, who do ship to the UK and might be a bit cheaper than suppliers over here (I am not sure). Otherwise, as MinimalistMommi says, Soulpad have a good reputation. I think Karma Canvas too are quite popular.
I bought mine at the end of last season to avoid any 2013 price rises, did exhaustive research over many many weeks and ended up with a Soulpad. Zoe at Soulpad is one of the most tolerant helpful customer service people I have ever dealt with, whereas the people at Belltent never replied to a single query. I was tempted by the Karma ones as last years model had a fly sheet inner door thing which the soulpad didn't, but they were out of stock in the end.