The regs do allow it. I had a lot of trouble getting it done though as it's so unusual.
You need it to be in its own fireproof room. They were mostly scared that my car in the garage next door might set fire to the tank though!
I like the idea of putting the tank in a locked building where neither thieves nor UV can get to it.
I dare say I'll have an argument with the insurance company in a few years when they say "The tank needs replacing" and I say "Why?"
Andy