If you compare this adaptor rig to a standard fill rig sold by planet at £49.95, the £25 doesn't seen too expensive.
The trouble with dive shops is, they don't need this adaptor for their primary source of business "diving". They're not concerned with paintball, it's us who are encroaching on their world. If this adaptor was part of their range, i guarantee it would be far more expensive to buy than £25.
The other option, is to buy a dive bottle and a fill rig, then get it filled at the dive shop, which works out at...
dive bottle, second hand = £100
fill rig = £49.95
Fill at dive shop = £3
That's over £150