If you go to Bristol or OATS for the DL you will get a good service from both of them. If you put the work in, you will pass all 14 subjects. So then, how do you select one over the other? It would come down to things like which one is the easiest to travel to, is one nearer to you, cheaper, or does one feel better to you when you speak/see their representatives.
My choice was easy. BGS were prepared to meet with me in the week I was available, Oxford were not. BGS then explained about the fact you could email your test results, so I was sold. But, I reckon the teachning and standard is similar between the two. You might want to check out GTS in Bournemouth as they seem to have a good reputation.