Gas cylinder mounted on vehicle - is this legal in the UK?

Does anyone know what the deal is with mounting gas cylinders on a defender?

I am hoping to attach to two Calor cylinders over the two small rear windows as is common in South Africa, but I do not see anything mentioned about this in the UK which seems to suggest this is not an option...

Would it be more legal to mount it on top of the roof rack ?

I am very interested if someone knows the legal situation about this in the UK.

In the uk you can not have Jerry cans mounted on the side so It should be the same with the gas but I think you will get away with it on the roof rack

Ps I'm not 100% sure on this so wait and see what other people say

I suspect it will boil down to safety... if you strap it on then no.. but if it is in a cage which will protect it thats a bit better. I know folk on the ferry had to take their gas tank off the roofrack... but it was held on with a seat belt..