.12's are not polished or go through a quality control process. If you have a look at .12's, normally the yellow or bright orange ones, you can see seams, bulges and knobly bits. All of these will rip your hop rubber to shreds, cause jams in the barrel due to irregular sizes and genrally be crap. If you have a large amount jam in the barrel/hop area then it can break the piston and bugger your gearbox. If you manage to find a polished provider of .12's then the fps of a proper AEG (280-340) will cause them to fly skywards, disintegrate or generally be highly inacurate.
£25 inc p&p is a bit high, but some BB's are £15-20 for 5,000. I know that I pay £7-10 for 5,000 and am perfectly happy with their performance.
You have not been ripped off, you have paid the going rate for that make. However there are other makes who are just as good, and a lot cheaper. Don't feel ripped off, they will be great. however have a chat with guys at your skirmish site and see what they use and recomend.