I've ended up buying an off-the-peg dress online (from Net-a-Porter) because I didn't want anything too 'bridal' and some of the boutique prices were very high. I'm really happy with it; I had a 28-day return window and took it to a seamstress to check that she could alter it as I wanted, which she can, and the alterations will cost around £200.
I tried on some dresses at Wed2Be and thought they were a great option - very helpful and lots to try on. The store I went to said they had seamstresses they recommend/work with a lot which is great. The only downside for me was that the store just has that stock you see on the day - there is a big range online but they won't ship to your address, nor will they transfer a dress from one store to another to try on. So if there's one you find online and set your heart on, you'll need to ring them, find a store with stock, and then go there to try it on, which could be quite far away!
Boutiques like you to order the dress 6 months in advance so it feels like now or early in the new year is a good time to look for off-the-peg, and then you have time to order one if you don't find one you love.
Also, if you are considering off-the-peg, don't forget the high street - my to-be sister in law got a Phase Eight dress knocked down from £600 to £250 in last year's January sale and it was lovely. Debenhams, Phase Eight, Monsoon, Ted Baker et al all have good options depending on your budget.
Hope this helps!