The Sigma 12-24mm f/4.5-5.6 has a large angle of view (122 degrees super wide angle of view at 12mm along the diagonal axis) and is ideal for shootings indoor as well as landscape photography.
349 on the site - 20 pounds off with voucher DISCOUNT20 and don't forget Quidco too (3%)
This price is 90 pounds cheaper than the next cheapest competitor - Amazon. Even at the famous B&H in New York, this costs $779 - which is about 450 pounds at todays exchange rate - and doesn't include shipping/tax/vat etc.
A remarkable deal if you are looking for an ultra-wide lens. The lens gets very good reviews, and unlike the Canon 10-20, is suitable for both full frame and crop cameras.
You can read user-reviews here:http://www.fredmiranda.com/reviews/showproduct.php?product=184&sort=7&cat=37&page=1
Delivery is free.