The Harwich to Hoek overnight ferry is very poor.
Really? i had a great experience on it not that long ago.
The problem with the Esbjerg connection is that you lose an exceptional amount of time and money to not get that much closer to your destination. Looking now -
Harwich to Esbjerg - 18 hours 15 minutes.
Esbjerg to Wroclaw - 932km
Harwich to Hoek Van Holland - 6 hours 30 minutes.
Hoek van Holland to Wroclaw - 1029km
The Harwich to Hoek overnight ferry is very poor.
It does the job if you just want to sleep overnight. The boats are dull, but functional. Nothing wrong with the facilities though.
The useful thing with the Hoek connection is that you can take it as a day ferry too and then do the driving at night - and of course, if you miss the Hoek ferry, you're not waiting 2 days for the next one.
I like Wildrover's suggestion of Hull-Rotterdam too though - Stranraer to Hull is a doddle of a drive.
If I recall correctly, there are special fares for doing Belfast-Liverpool and then Dover-Dunkerque by ferry too as it's the same operator. There may very well be some sort of promotion on Stena too.