"Britain has a surfeit of gas"
Hahahahahahahahahahahahahaha.
Britain has no gas.
Companies some of whom are British (for tax purposes) have gas wells in British sectors of the North Sea.
Which they sell gas out of at prices set by the world market in $ to whoever they can legally trade with.
And the UK has been (IIRC) a net importer of gas since IIRC (can't be arsed to google it) 2011.
While I'm delivering a little economics lesson Sunaks "Windfall" tax was FA like a real windfall tax, which is a retrospective tax on profits companies have already (and unexpectedly) made.
Now, you were saying....?