'20% of the nation's gas will pass through the PRI'

- true (ish)

'therefore, without the PRI, the nation will be denied 20% of its gas'

- false!

The pipeline is already operational at 75 bar and could carry about 16% of the nation's gas. A PRI would enable it to operate at 94 bar, thereby increasing its capacity and permitting 20% of the nation's gas to flow. The effect of the PRI would then relate to 4% of the nation's gas, not 20%.

An important distinction.

NG always uses the first statement.  Could that be because it is the larger figure?

… and  perhaps I should also mention that in addition

up to 50% of UK demand can be supplied by the North Sea, and

up to 105% of UK demand can be supplied by non-Milford Haven imports, and

up to 25% of UK demand can be supplied by projects now being built, and

up to 25% of UK demand can be supplied by projects now being planned.

all of which means that, well, we can transport enough gas for as far into the future as anyone can see.

And all this against a backdrop where the government would like gas demand to fall by 20%!

The Department of Energy puts it this way: 

“the UK has a robust and highly resilient gas supply”.