Putting it in basic terms I can buy all the parts to make bombs legally over the counter, but does not mean it is legal to makethem. Also we can all buy electrical gear, but it does not mean we can legally wire our homes for electric (unless you are an electrican).
Have your own dive bottle and filling it will not solve the problem as you then have 2 items that can got bang. The chance are your Dive bottle will be legal as dive shops are normally very strict on these things. But if your bottle is not legal and goes bang again on someones site, the HSE will once again close site till they are happy.
One my team mates bought a reg (low pressure) from Gizmos and it exploded while filling up from a H Pac system. H Pac weren't to blame nor was the site and luckily no one was hurt. Although the parts did fly closely by the guy from H pacs head (not sure if he knew how close). Anyways the point is the bottle had the reg fitted properly but it was not checked. If it had been checked my team mate would have known that it was not feeding 450 psi it was in fact feeding what ever you put into the bottle. So in the end his angel blew all it's fittings at about 2000 psi !!
He was lucky no one was injured and he will always have every reg he has fitted checked for pressure out put now. He has full UKPSF air training now and he carries full insurance from the UKPSF. A close call like that made him wake up and smell the coffee !!
Go to the UKPSF website and look for their air fill policies or the NPC and see that all their sites have to run on the UKPSF air fill policy.
5000 psi in a bottle is a bomb if it is not fitted and looked after. I personally thank the UKPSF and NPC for bringing these problems to light before someone is hurt or killed and the good name of paintball is dragged down.
Also remember on the time scale of paintball high pressure fills are a pretty new thing. I remember the old days playing with just eye protection, now i'd never play with out the best mask I could get for protection.