transit – Is 2 hours 50 mins sufficient in Heathrow to connecting flight with different tickets and airlines

If everything runs well you might make it, if only one thing takes more time than planned you will miss your second plane.

It usually takes between 30 and 90 minutes from the plane seat to outside the luggage collection area. Most airlines ask you to drop your luggage at least an hour before the flight but longer time spans are possible. And if you drop your luggage at the last moment you will have to be very quick through the airport as you will have no spare time.

You see you do not have a lot of time if things are a bit slow, no time at all if your route through the airport to your luggage (or luggage delivery) takes a lot of time.
And that is assuming it is in the same terminal. If you also have to move between terminals, forget it. No way possible at all.

Usually people advice 4 hours or more between flights if on separate tickets. But with a long flight as the first of the set, I would want more time.
It might be needed to buy a second ticket to Helsinki, or at least move your flight by 4 or more hours, 24 is more likely.