The number on the bar codes needs to be licensed from GS1uk, who are the company which manage all the numbers. Steve has mentioned their website above. It is a good idea to read about this prior and be sure it is something you need, as you will have to continue paying annual fees for use of the numbers. For small companies this can be costly and unnecessary (as i found out!).
Once you have your numbers, you will have to approach another company who will produce the actual images for you in the required format (either as a file or labels). So there will be this additional cost on top of the licensing fee you pay.
If you need specific help just let me know, as I have been through thr process so would be happy to lend a hand!