Personally for anyone wanting to fill there own ask for diving O2, it makes about £3 difference for a J cylinder and is certified as a breathing gas.
Do you have any names of dealers who will deliver to a domestic address, and charge about £23 for the full-size bottle of diving oxygen?
Getting o2 delivered is actually hard, I found, because the dealers claim their lorries won't carry o2 bottles on the same lorry as other gases. I live across the road from a pub which gets co2 gas deliveries all the time, but the company said it's far too much trouble for them to deliver o2 separately.
The good thing about welding oxygen isn't so much the price - there is no practical relevance to paying £20 v. say £60 for a huge bottle capable of doing one's entire annual airways flying requirement - it is the fact that welding supplies dealers are on every corner and it's easy to drop by with the old bottle in a trailer and collect the new one. I dread to think what my car insurance company would think of it
but the welding gear dealer doesn't care; he gets all kinds of body repair cowboys dropping in all day long.