Asda have a number of Wii compatible accessories instore at £5 or 2 for £7.
They're mainly by "Gamexpert" and include the Venom VX Shooter, a Sports Resort Pack, a Sports Pack,
2 types of Steering Wheel, and a wireless sensor bar (useful for wall mounted TVs according to the packaging - though there is a better deal for a wireless sensore bar elsewhere).
They've also got the "Blue Ocean Sports Pack 28-in-1" (black or white) for £10.00
This is on Amazon as per this link:http://www.amazon.co.uk/Blue-Ocean-Accessories-Sports-Pack/dp/B001G7PBY6/ref=sr_1_9?ie=UTF8&s=videogames&qid=1278013203&sr=1-9
I spotted this in the Queensferry (North Wales) store yesterday, and although I think it's nationwide (as they were marked at these prices on the shelves), I can't confirm this.
EDIT: removed wireless sensor as there is a deal on here that is better and I'd only added it as an example, having bought the Sports packs and a couple of guns for myself