Sounds feasible to me.

It's a non-Schengen --> non-Schengen connection.
The LHR flights usually arrive at the F-pier, not sure where the LED flight leaves from but the probability is quite high it will be from the F-pier as well (I think I can recall having seen an F-gate for the LED flight before), then it's a piece of cake...

International MCT at ARN is 30 mins and it's usually fine
I would be more concerned about traffic delays at Heathrow than the actual transfer at ARN.

There is only one daily departure on SAS to LED, so if you miss that one you have to check alternative routings.
From ARN I think Pulkovo also flies to LED, but I don't know about the timetable.
SAS flies CPH-LED, otherwise it might be possible to find a connection via HEL, TLL or RIX.