In reply to mypyrex:
Men's too, as Indy suggests. Getting a really nice dress made from scratch by someone in the UK is going to cost a bomb, and local tailors can be a cr$p-shoot. So much of a dress is in the fabric too, which is often much harder for a small tailor to get right.
My wife and I always seem to do OK buying the nicest clothes we can 'sensibly afford' and then get them adjusted (often quite a bit) by a very good local tailor. When we got married my wife didn't want white, so wore a dress from Liberty (very fancy but not the normal wedding dress second-mortgage affair) that had been fairly extensively recut.
I reckon buying something relatively classic in the £250 range (Shop will depend on taste, figure, age and all sorts of things, but maybe Jigsaw? J-Crew? Karen Millen? Hobbs as mentioned?) and then getting it altered by a good tailor might be the best bet overall. That also gets away from nagging fit issues so there are loads more options - as long as the neck/shoulders and any really defining or structured elements of the outfit fit correctly, the rest can be sorted by a good tailor. No more rejecting a lovely dress because the waist/bust/hemline/whatever aren't quite right.
Alternatively, just a fairly standard suit or dress could look great if you spent the money on some seriously good shoes or a very fancy scarf/hat/necklace? For £350 you could get one or two really amazing accessories that would also have a usefulness beyond the event?
Happy shopping.
Post edited at 19:12