Looking at the Ri instructions it says that the gas connection is 22mm, this would mean that a 22mm supply is needed, as you cannot reduce the inlet supply
I dont see how you can logically come to that conclusion!
It is as perverse as those who think because Vaillant have a 15 mm gas inlet the whole gas supply can be in 15 mm.
The gas supply has to have a minimum pressure drop of 1.0 mB. Thats the only criteria.
Many makers say the gas supply has to be 22 mm. Thats only because otherwise cowboy installers would do it all in 15 mm.
It says in the instructions that the Gas connection is 22mm, now I don't fit Worcester so I don't know, but if the gas connection is 22mm then it needs to be connected with 22mm, do you think your Gas Safe inspector would except you connecting a boiler with 15mm when it has a 22mm connection at the boiler, I don't think so