I definitely agree on going to the MB Spa after you land to put your name down and then go into London for lunch. I have down this in the past.

I think the way I did it was follow the signs for T4 transfer and you go through security. Remember where you come up after security. You need to go back to this area again later. Then you can go to the Spa. After that head back over to the other half of the terminal which I think are where gates 1-7 are. On the left side (near where you came up earlier on after security) there is a lift that takes you back down to the transfer area. You come out of the lift and go through the turn doors and you can then proceed through immigration.

If you want to use the Arrivals Lounge you can definitely do that anytime of the day. Just show them you boarding pass.