If price is really important Hertz and Avis won't be your first choice...

I rented from Dollar at LHR and dropped the car off in GLA with no problem or surcharge -- this was several years ago, but it was a LOT less than $540. Got a midsize Peugeot that seated four very comfortably.

I have also had good luck and great rates with Kemwel and Woods, which have off-airport locations around LHR... maybe in Suffolk too.