Northolt has taken 737's in the past - although not with many passengers, I suspect.
There are all sorts of things that could be done rail-wise for Heathrow. Any connection into the main rail network would be a good idea!
Heathrow's short does get used, when there's a strong southerly, which makes the approach really uncomfortable, but there you go.
Why, why, why, why, why don't we use the lovely long MOD runways more?