The poundland ones will probably be fine. As long as they hold air they are perfect!!
Wilkinsons do them for £2 each but I've not been in for a while so that might have gone up.
If you have one of the "one day sales" on near you they normally do bundles of tubes pretty cheap.
Asda/Tescos, use to buy GT tubes from LBS (cos they came in a pretty
box !). Haven't repaired a puncture since getting back into cycling 13
years ago (did enough in my teens !).
Go for cheap as chips and carry a couple of spares and a pump.
Asda is good for 16" tubes for anyone with a kiddie trailer !!