B2B at TLL is foolproof if HBO. Arriving and departing pax are in the same area. Get off the ATR, walk up the stairs, and you are at the gate where the pax for the flight back wait.
If you miss the HEL-TLL leg, ARN-LHR will be cancelled too, assuming this is on the same PNR. Worth noting, in case you are unaware, the ATRs board from the back.