Hi and welcome,
The internal diameter of gas line is about 1/4" or 6.35mm. I've a barb on one of my fittings which is slightly larger than this to stretch the plastic line and make a tight fit, I just measured it and its about 7.2mm diameter.
This is for the 3/8" gas line that links the corny grey disconnect with the regulator's low pressure output (not something that fits direct to the high pressure gas cylinder, e.g. to connect a remote regulator). My regulators have all had 3/8" John Guest push-fit fittings on already (hence I've no idea if your 3/8"BSP is correct or not), so thats an alternative to a barb if you want one.
Cheers
Kev
Fitting a Regulator