Embrace bridal boutique in Deeping St James (just north of Peterborough)
I have ordered my dress from there - all samples are 16+ and they stock some specific plus size ranges too. Great customer service. Much better than all the other shops I went in and it was so much nicer when you aren't squeezing into things three sizes too small and feeling like a great big heffalump.
Most retailers and designers will go up to size 28-30 It's finding them in the sizes that's the problem so would be worth having a look online, finding a few that you like and calling round all of your local stores to see who stocks them in a size closeish to you
And get a fab seamstress, the most fantastic dress will look appalling if it doesn't fit you exactly
Bit further than you want to travel, but there's a place called Heavenly Bodies in Catford (S London) that gets outstanding reviews & recommendations.
May be worth looking on the You & Your Wedding forum pages and asking on there, as you're likely to find more traffic there and it's something that is regularly discussed - there's a specific page for fashion and another for plus size brides, either would have plenty of traffic for you to get recommendations closer to home if you don't want to travel as far as London.