Depends what your looking for I suppose??

Have you looked at cruises from Southampton

NCL Sun Baltic.

Informal options very thin on ground, especially now that NCL Jade will be out of Barcelona or Palma.

I can confirm that NCL is exceedingly informal. We were on the Jade this summer and nobody turned us away from Cagneys when the children (10 and 15) insisted on wearing their bedroom slippers. Perhaps it was not so noticeable under their trousers, or nobody ever looks at kids' footwear. Same comments with the MDRs, Blue Lagoon and the sushi bar.

But there is a dress code. I did see a person turned away from the buffet for coming in barefoot.

Update on RCI.

No problem casual on the RCI Independance 13 TA.

Plenty of people not dressing up and eating in the dinning room even on formal nights

I had reef sandels, jeans, tshirt/polo most nights , put shoes, black trowsers shirt on for the formals.