I have just been re-reading about my Big Day Out to Saltaire, and I realise that, by the same token as I have been to Bristol - i.e. being dropped off at the station - I have been to Bradford. For what I had forgotten was that after attending the organ concert, and then going for a bite of tea in Shipley, I had missed the train from there, and David gave me a lift to catch one at Bradford. Sadly I did not take a photograph of Bradford Interchange in the dark. So, Hull is absolutely, definitely and unequivocally the largest town I haven't been to.
I recall that when I was at Huddersfield, there was some discussion about whether it was the largest town that wasn't a city - with plenty ready to claim that it was. The ONS figures DG cites suggest, however, that - at least now - it is vary far from it, being 11th in the list once cities are discounted.
And on that basis, the biggest town that isn't a city is in fact Reading - the putative destination for my next Big Day Out (along with Ladybank, when Edinburgh start stumping up for me to attend exam boards again). At present I am averaging one Big Day Out every two years, so at that rate I should have completed them all by 2385. Perhaps I should hurry up a bit.