It appears that Heathrow has made another service available which totally misses its real problems. Following hard on the heals of special training for its staff to cope with Imaginary Friends, they are now arranging for passengers to terminal hop, to shop.
The Heathrow web site describes the service as follows:
If you have more than three hours before your flight, we may be able to arrange to transfer you to another terminal to visit shops not available in the terminal you are travelling from.
To use the service you have to commit to spending £250 and you have to pre-book the service. There is even a special online form for this purpose.