I have tried SAS Plus from CPH to IAD, EWR and ORD. I find the product solid and would not hesitate to book Plus on any of these routes again. There is enough space to work with your lab-top and sufficient room to stretch legs. The food is hit or miss.
Also, I would expect LHR-ARN in Plus to be more comfortable than LHR-FRA in LH regular economy. I have tried LHR-CPH in Plus which was quite decent.