It doesn't sound like you need much - certainly not a £75 race spec lipo as suggested. As you don't race, there's little point getting a lipo on the BRCA homologation list, as they're always so expensive.
I would advocate a hardcase, though - lipos are well protected in a B4, but it only takes one freak occurrence to cause a puncture and thus disaster.
A quick search of ebay found this for under £30 delivered (click photo):
Vapextech are UK based, reputable, and supply BRCA listed race packs (probably not this one though). 5000mah at 30C is 150A peak output, which is way more than you are using!