Can try Kelly's Bridal .. They have a reasonable selection of gowns for my wife. My wife is not exactly plus size, however, she was 7 months pregnant. She still managed to choose a few white gown and evening gown from there.
If you like ball gowns, The Aisle bridal has a very nice range of plus sized white wedding ball gowns. I'm not sure what the AD only packages are like because I signed on for a full prewedding package with them. You can also consider Jess Haute Couture; their OTR gowns come in sizes up to UK20 and their MTM prices are pretty reasonable as well.