Poundland have inner tubes, available in 3 sizes: 20 inch, 24 inch and 26 inch.
When I was doing a 40km commute on my bike everyday I got the Marathon Plus tyres after getting 3 punctures in 1 week, had to mend in pouring rain too. No punctures for a whole year! I have many years of experience of patching tubes though and they have very rarely leaked on me. I do carry a tube as it's usually quicker than patching but you still need to check for the source of the puncture in case the glass/nail is still stuck in the tyre, usually is.