The guidelines say that it should be a solid brick or stone wall (including breezeblocks). No mention of it being an external wall, though a supporting wall on a modern house is probably your best bet, as the others can be pretty flimsy!
You CAN attach it to a plasterboard stud wall, but you'd probably need to plate the opposite side with a metal sheet to give a suitable fixing.
The best person to advise is your FEO, as they'll know what they are willing to accept (should be standard, but blatantly isn't!) If you think your security meets the legal test (a "reasonable standard") but th