Argos have the best deal I have seen so far
£229.99 but you get an accessory Kit (£20) plus £60 worth of vouchers (effectively meaning it costs £150 for the console, or £127 for myself thanks to a familial discount :D). They also have another option for £259.99 with the same plus a game for £30, £9 off, but you are limited to a choice of Super Street Fighter IV, Pro-Evo Soccer or Nintendogs and Cats.
As far as I found Shopto has the cheapest actual price in Britain at £199.85.
Personally I wasn't ever intending to pre-order or even get the 3DS early in its life due to its prohibiting cost, but with the Argos deal I would be able to get it for, effectively, almost as cheap as I did the DS Lite, so I will be in the next day or two.