The taxes returning from the UK are more than double compared to returning from Poland, as you can see in the screenshots above.
Any interest in flying into EDI but returning from WAW during the same trip?
The reduced taxes returning from Poland would likely pay all or most the EDI-WAW one way fare on KLM.
You'd have to see if adding it as a leg to the trans-atlantic ticket or buying it separately as a standalone ticket would be cheaper.