I share your concerns - but never forget, most batteries are made in China. It's very likely Dell branded ones are made there. If you buy from a reputable-looking outlet in the UK, chances are that the battery you get will have been made in China. At the end of the day, there is always a risk in buying a non-Dell (for example) branded battery - but you pay a huge premium for 'the real thing', as I'm sure you know. FWIW, I've bought several cheap 'off brand' laptop/netbook batteries and been lucky - but I'm sure others have had bad experiences.
But what as already been stated, most batteries now originate from China or in the same area, and in the main possibly from the same production line of some of the major battery manufacturer's, as re-branded versions for well known named clients.
The idea of checking reviews might resolve any issues regarding reliability of supplier and product!.