Only options I can think of, unless I've got this completely confused,
are to build out the wall with a sheet or two of plasterboard, OR,
fit a high level cistern that allows me a further distance from the
wall for the pan.

I'd install a concealed cistern toilet. You build a cabinet at worktop
height exactly as far as you need to install the waste. The cistern goes
behind. The toilet backs on to the "wall". You use kitchen or bathroom
worktop on top of the cabinet. It is cleaner and neater and you gets lots of
worktop space. If it is wide enough, you can install a semi or fully
recessed basin in there too. I prefer semis myself, as the "rim" is far too
wide on a fully recessed.

Christian.