There is an obvious solution. Build a transit system between all the Heathrow terminals and extend it to Northolt. Add a station where it crosses the Great Western Main Line. Run a combination of "airside" and "landside" transits, as they do at Frankfurt. Sure it would cost a lot but probably no more than projects such as the Heathrow Express, and it would remedy Heathrow's greatest weakness - as ConstantFlyer says, it is three airports (four if/when Heathrow North gets built). You could also move car parks away from the Central Area.
As an aside, there is a cargo tunnel between Terminals 1 & 4. I often wonder how much it would have cost to have engineered it to run passenger transits. OK, its not really needed now BA is a T5 plus a bit of T3 operation but it would have been useful when they ran a split T1/T4 operation.