Bradford league used to have a rep as the best in the UK when I was back home, but thats 15 years ago now and I always suspected that some of that rep was self promotion on a massive scale. We were in the York Senior and the standard differential between us and Bradford League clubs when we played them in cups was not massive, though they did tend to get the best pros, and to pay more players than any other league I'd seen until getting down South.
From what I hear, the Yorkshire Premier has taken a few players away, certainly in terms of the lads with county aspirations, and my brother swears blind that the A/W has improved markedly in the last decade, though that could have a lot to do with him playing in it! (his views, not the standard!)