Rule of 15's
If it's less than 15 yards OR less than 15 degrees - treat it the same as you would for a level shot.
If it's more than 15 yards AND more than 15 degrees - aim low but still inside kill.
That will work for 99.9% of the shots you're ever likely to see in the UK and for the times where it won't work, knowing where to aim won't be the biggest problem, it'll be figuring out how to keep your crosshairs still that's the biggest problem.
(as a footnote - if you're using a springer, all bets are off)