I did this (exactly this connection) recently. While it is indeed an efficient airport, it was quite crowded (you need to go through passport control, too, so potential queues) and walking from one end of the terminal [bus gates from TLL usually arrive near gate 22 and our flight to LHR departed from the 36H (another bus gate)] took a while. When we arrived at the gate, the first bus had already left. Boarding started 30min before departure, so effectively about 10 minutes after your arrival. Certainly no lounge time.

However, HEL-TLL is blocked at 30 or 35 minutes, which is generous. Usually the flight is just a mere 20 minutes and taxi time from/to runways in HEL (no full length takeoff required) and TLL are very short.