You really don't need it, unless you sending a lot of fish a long, long way. What you need is large enough bags, and you can use an air pump to fill them up with regular air. Should be at least 2/3 air, and enough water to cover the fish no matter how the bag is lying.
Seal up the bag tightly, and if they will be in it long, add a bit of Prime or salt. Prime will neutralize the ammonia.. salt in the right proportion will inhibit the uptake of nitrites into the blood, preventing nitrite poisoning. Commonly done by commercial shippers.
Also, double bag and maybe some packing tape across the bag bottom, to prevent any 'corners', where fish can get stuck. If it's fish with spines like a cory, triple bag.
If the UK is anything like Canada, you might not even be able to get oxygen, unless you know someone in a clinic or something. Here, you need a prescription to get it unless you are a fish store and buying huge tanks of it for that purpose, and I haven't a clue how that's regulated, though I am sure it must be. Anything involving pressurized containers is usually regulated.
So don't worry about O2.. worry about good packing and plenty of air and maintaining as even a temperature as possible. Foam insulation works pretty well, and you can get heat packs if need be.