Strangely the web page still quotes the opening hours being 0830-2200, but in reality it is open already earlier.
Due to the move of Finnair operations to T3, Finnair passengers now use the Cathay lounge in LHR. As the first Finnair flight leaves at 07.30am, the lounge opens now early. I was there already at 05.50am, and I was not the first customer in the lounge.