Based on actual watches I own, or have owned
TWCO is the best quality I have seen for this price range in a long time; it comes in well below your budget at £530 (including VAT). May be a tad large for you though (43mm), although it wears well. Extremely solid, and well accessorized and boxed (Peli case). http://www.twcowatches.com/
If your budget stretches to £800 (£600 without VAT) and you could find one, the Crepas Cayman 3000 is hands down the best quality watch you could find in this price range that easily challenges £5,000 diver watches, heck practically any diver's watch for quality. Yes, I also own Rolexes (new and old) and most of the usual suspects; I am not making this comment lightly. Perhaps a used one may be cheaper, although I doubt they are discounted much due to no one wanting to sell one once they get one :D Not that I usually care, but the box is also very nice (quality glossy black painted wood) and comes with a generous selection of spare straps and tools.
Deep Blue watches are also excellent, although some are very big in size. Also very solid :)
Christopher Ward Trident: Elegant and refined, but perhaps a bit more "fragile" feeling. i.e. I wouldn't take one to the sea even if it will work just fine; it's more of a suit and tie type watch in my eyes.
Steinhart Triton (I have the bronze one): Solid watch; horrible strap, but once swapped out it was good.
Magrette: Very original designs, well below your budget and good quality build. Strap deployment a bit finicky, but overall really hard to pass up on this brand.