How about something like this:
I haven't tried/used it but just did a search for "free point of sale" on google.
The POS interface looks fairly rudimentary but it may do:
And it is open source so in theory you could customize it if it falls short.
It is more of an accounting system with a bolted-on POS interface though it seems.
I think to develop something you'd have to have pretty good developer skills, an alternative would be to buy something but I'd imagine you'd be looking at 2 to 400 pounds, something in that region. e.g.:
The nice thing about those guys is you can buy the bits like chip & pin readers, barcode scanners, printers etc.
Hope this helps.