I found Gamestation to offer the best deal for preowned Nintendo Wii - my local Gamestation does this for £79.99 including ONE YEAR GUARANTEE (same as manufacturer guarantee).
Online this price is £89.99 though it doesn't specify the one year guarantee on the product page, the help page specified this in regards returns:
"Faulty hardware may be returned for a like-for-like exchange up to 12 months after the despatch date. "
See here for details: http://www.gamestation.co.uk/Help/Returns/
In-store you can also ask if there are any copies of Wii Sports available (the game which usually comes shipped with Wii). It's apparently not bundled with secondhand ones by default, but if you ask staff may try to find you a spare copy as they don't sell them seperately.
I was also able to get a copy of Let's Dance 2 for an extra £10 on top of my purchase (saving £19.99 off RRP). I think this may have been a local instore promotion, but is well worth a look in your local store.
In any case, I'm well pleased with my savings - nearly half the regular price of a new Wii + 2 games - and hope this will be useful for someone else too.