Do i really need a transit visa for travel from EU country to Bangkok via london?

GP deserves the 10 Points. I cannot see a problem as you have Dannish residency which is part of the EU. You should have enough time to pick up your luggage from terminal and then proceed to terminal 3 for Thai flight but I suggest you do that straight away as security at Heathrow is very Tight and can take time but 3 hours will be enough for you to make connection.