How to use ABC Extended with standard British PC keyboard layout

The ABC Extended input is fantastic for inputing several diacritics, but it only seems to assume apple's own layouts, i.e. it doesn't recognise the standard British PC layout. Is there a way to make it work with the standard British PC layout, particularly re: the symbol mismatches between the two?

Standard British PC has shift+2=" and shift+'=@.

The Apple British layout has shift+2=@ and shift+'=".

In British PC, the key left of 1 is ` ¬, but in Apple British it is § ±.

In British PC, the key left of z is \ |, but in Apple British it is ` ~.

In British PC, the rightmost key of the asdf row is # ~, but in Apple British it is \ |.

Suppose I definitely do not plan to use the Apple British layout at all, but may need to work with the standard British PC layout. Is there a way to reconfigure certain things for the ABC Extended input to be compatible with the standard British PC layout?

Update -- Problem solved with Ukelele. I copied the layout of the current input source and then manually adjusted the key mappings (including the shift modifications etc.). It took me just a few minutes.