shops seem to sell some metric, some imperial. a rough calculation makes them seem almost the same but the product descriptions don‘t mention it being the same as an 8mm fitting. this is for a gas fitting in a camper.
If you are using compression fittings then you can obtain special thicker olives for the fittings. Check here: www.plumberparts .uk They seem have a pretty good idea about compression fittings.
For some reason the caravan/boat world still uses imperial fittings and pipe sizes where the plumbing trade certainly in the UK is all metric although the threads are still imperial 1/2 3/4 and 1 inch BSP etc so no really you need to match the pipe sizes to the fittings to ensure gas tight and mechanically sound joints.
