Sorry to burst the bubble , but for the next few months or so it won't be that easy ........

You will arrive at Terminal 2 , you will be given a fast track card , if you dont get one then ask for it , on arrival you will pass through the immigration at the end of the terminal , do not join the standard queue as at 5 PM when the LHR flight arrives it can be quite busy .......

It shouldn't take longer than 15 mins to get through to the baggage hall , not sure if the baggage is connected or you will have to pass customs at Jakarta , i believe it will be connected , so you should be out of the terminal in 30 mins max

Now you will have to change to the new Terminal 3 , all domestic garuda flights have been moved there , whilst international remain in terminal 2 , Garuda have arranged buses for the movement of passengers , this shouldn't take longer than another 30 mins

Its do able but it will you need to be quick , i would hope Garuda have arranged everything smoothly