From where in Poland to where in the UK? This will make a difference to your route.
I've ridden to Poland three times, taking different routes each time. For preference I would keep south, say leave Poland at Nachod, along the northern edge of the Czech Republic to Cheb, then head west through Oberpfalz and northern Bavaria towards Schweinfurt, then skirt north of Frankfurt towards Limburg, then down the Lahn to the Rhine. If you are heading to Calais then cut along the Mosel to, say, Cochem, then straight across Belgium, if to the Hook you will have to head up towards Aachen, but I don't know that area.
This route should avoid most of the major conurbations and take you through some quiet and scenic countryside.
If you want to avoid the Czech detour you could cross straight into Germany at Goerlitz and across Saxony. I wouldn't go north of this: northern Germany is not as scenic as the south and can get very windswept.