presta
butyl
23mm
30-40mm valve
Evans used to do some good deals a while back.
Cheapest I've found so far (quick lunchtime search...) is 2.50ea
http://www.evanscycles.com/products/onza/sa3-innertube-700c-presta-valve-ec034309
Cheepest are decathlon, used to be 2 for £2, now £3 IIRC, but quite heavy and I'm sure they're draggy, fitted them to my inter wheels as they were all that was in the shed and I'm sure I'm now at the back of the group dying rather than comfortably keeping up.
Middle of the road conti/spesh tubes 3 for a tenner.
Michelin lightweight are £5 at decathlon too and seem to be the best comprimise between cost/performance (i.e. they're £1.60 faster than the normal tubes if such a things quantifiable).
Nice one
spotted this 5 x Vittoria for £15
http://www.ebay.co.uk/itm/Pack-of-Five-Vittoria-Ultralite-700x19-23C-Presta-Valve-inner-tubes-/290911568873?pt=UK_sportsleisure_cycling_bikeparts_SR&hash=item43bbae43e9
but the wiggle one looks like the winner