I use Hobby King for lipos for my helis. Frankly, I find Overlander and other UK outlets extortionate for lipos but each to their own. Make sure you shop from the UK or EU warehouse. If you use the Global warehouse, you will have import duty and Royal Mail handling charges to add on.
Turnigy Nanotechs do tend to 'puff' a little. Mine are now over 5 years old and have lost a little power but still good to go. Then you have Turnigy blues, Zippy Flightmax and Zippy Compacts (yellow). Hard cased packs are for cars to stand the collisions and impacts while Zippy Compacts are a little narrower than the rest where space might be tight.
I dont use Graphene packs. I understand you can supposedly charge these on a good lipo charger as they are supposed to be Lipo cells but couldnt vouch for that. In reality these pack more of a punch so Id say only buy these if into racing.
Overall, if space is tight, buy Zippy Compacts otherwise Id recommend Zippy Flightmax for better performance or Turnigy blues which are a little cheaper.
Depending on what your use will be, you shouldnt need anything more than a 25C rating for general scale. If you want fast scale for a heavier boat, probably a rating up to 45 - 50C will be adequate. If you are racing, you will want something like 75C rating.Essentially, the more 'S' equals more voltage ie 2S is about 7.4v, 4S 14.8v, 5S 18v, so will give your boat a faster speed. Id say for a 1\12 Trent you'd probably need at least 5S and maybe even 6S.
Put another way, for my large scale helis I use a couple of 5S 5000mah 25C packs. You are better buying for example two 4000mah packs rather than one 8000mah pack. Reason being an 8000mah pack will cost more plus if you get a dud cell in it, it will virtually render it useless whereas you have to be unlucky for a cell to go in both 4000mah packs.