Hyperion, Neu, PolyRC/PolyQuest and Thunderpower. Cannot go wrong with those 4. I've used all four too, and from my experience they are all solid, good performing packs.
The series to go for:
Hyperion - the VX series - for better choice of capacities, and for the higher discharge rate
Neu - the EP or NP - for better value, and still excellent performance. Just make sure you get the 'V5' ones, that allow a 5C charge rate - you don't have to use it, but it means it'll keep its value better if you sell it on later - people like higher numbers.
PolyRC - always expensive, but you really have to go for the new 45C range.
Thunderpower - the 45C range looks great, but, if more on a budget, go for the 30C range - still plenty of choice and power.
Best place to go for LiPos (aside from Neu) is
RC-LiPos in the US. They do free worldwide shipping, a bulk discount (if you buy 3 LiPos, you get 5 or 10% off), and try their best to prevent unwanted customs charges. The shipping they use isn't bad either - it's pretty quick. They are also about the same price as anywhere else, so you're not paying for the shipping through the price either.
Whereabouts in Kent are you anyways?