Rather interesting question however the answer is far easier than you make out. The reason being is that paper (inflationary) didn’t come about until WWI at the time £1 = 1 Gold Sovereign
In fact Gold sovereigns are still legal tender here in the UK so you are welcome to spend them in the shops but would still only be worth £1!
However for their gold value they are currently worth about £300.
So quite simply that puts Mr Darcy’s annual income at about £3million