It seems like you have a decent amount of time to research - which is going to be the key in your case as you have something specific in mind and also a budget to adhere to. A lot of people have suggested Ebay and I think thats a good starting place, but then you have the problem of not beign able to 'try before you buy' (and if you don't like it/or it doesnt fit you can end up having blown your budget or having to pay more for costly alterations). I suggest another tactic which is charity shops and second hand clothing shops. Oxfam (would you beleive it!) actually has a bridal department! See the link here for stores that stock wedding dresses - some of them are BRAND NEW so you could really bag yourself a bargain. I would suggest if you don't find a dress the first time you should tell the store manager what you are looking for and leave your details so that they can call you when more stock appears that would be suitable for you. Heres the link:
http://www.oxfam.org.uk/shop/highstreet/…
High street is your other option within your budget: BHS has already been mentioned and does have a good range. Debenhams also have a lot; the Oxford St branch has a bridal department and even if you get your dress from ebay/other then these two stores would be a good place to get the veil/accesories from. You don't say where you live but I would imagine that there is a local newspaper/buyer-seller pages where you can scan for 2nd hand wedding sales and obviously go see before you buy. One piece of advice, make sure that if you buy a second hand dress that it is already dry cleaned. Cleaning can cost around £100 (most dry cleaners charge a lot because its a specialist job) so you don't want to spend half your budget on that! Hope you find what you are looking for, I am sure you will! Good luck for the wedding - enjoy!