30, yes the "Gunpower Stealth" was the original and a UK design, then John (last name escapes me, Airforce fellow) started making them in TX, not certain of any business arrangement???? Tom Gaylord who writes a popular airgun blog in the US was instrumental in there gaining popularity in the US and I believe was an "airforce" employee when they first came out here.
watuse, pretty good bet NO ONE in the US has one as that model didn't come out until after the UK laws became insurmountable for those of us in the US, perhaps someone from the UK will chime in.
They do have a following in the US (Airforce rifles that is). Tinkerers esp. those looking for maximum power seem to enjoy them. There are several know drawbacks. The safety isn't. Briefly there was a US manufacturer making arrow barrel kits for them but stopped due to this issue and liability. Also they suffer from frame flex, hold it tightly ot press your cheek onto the bottle as hard as one might normally do on a standard rifle and the flex will through your POI off. The power adjuster is perhaps not the best at it's job, odd check weld, very high mounted scope, maybe a few other draw backs?
That said some folks love them. They did create home industry's for several folks making various parts to increase power, prevent frame flex , tuning, ..... .
If you are in the UK you very likely can not have one but you might get a kick reading up on the Airforce "Texan".
John