HAL will let you go to any terminal (airside) if you want to shop... (With a valid boarding pass for any terminal), you need at least 4 hours. Each time you change terminal you will be re-screened for security.

Though HAL have recently reminded all Heathrow lounges that they should only allow in people departing from the same terminal, this is due to T2 opening and T1 closing causing moves, and can walk between those two terminals (without search). How closely the lounges follow this is to be seen...