I was actually following this flight last night because I am flying LPA-HEL today and the plane from NRT was scheduled to operate HEL-LPA-HEL after your flight. I wasn't too worried, though, because it is an Aurinkomatkat flight so they will find a plane elsewhere, which is what they did, and are operating LPA as scheduled.

Your situation sounds stressful. I am a bit surprised that AY had not organized anything when you deplaned. Next time in a situation like that, just walk to a nearby hotel and take a room. No need to book over the internet. You need to pay for the room but as long as it is somewhat reasonably priced, AY will reimburse. No need to spend the night on the terminal floor. Calling Helsinki will not help, they are not able to arrange accommodations and will hardly know anything more about the delay than what Flightradar etc. can tell you.

The departure at 4 pm is likely due to crew rest. It is a very long flight, so they cannot have the crew up all night and then the plane leave at 6 am or whenever the curfew is lifted. You are on your way now, and once you land in Helsinki, they will have a new flight for you to London, and will arrange accommodations at an airport hotel.

Whether you are eligible for EC261/UK261 will be a complicated matter. Technical problems are eligible, but a nightly curfew by the airport is not something that AY can control. Once at home, fill in the form (it is very easy) and see what AY suggests.