Chesham Grammar has historically admitted children from Harrow, and the transport link is the Met Line from Harrow-on-the-Hill, now a direct line to Chesham. (It used to be a shuttle service for the last leg.)
The fly in the ointment is that CGS is increasingly popular, and the allocation distance is decreasing all the time.
There are effectively no other Bucks grammar schools that it is possible to reach from Harrow within a reasonable journey time or where you could reasonably hope to gain a place in any given year.
Take a look here to get a better idea of the lie of the land: viewtopic.php?f=12&t=41421