I'd thought of Northolt as a new EGLL runway too. However, my thoughts were to use it either for slow traffic only, or traffic below a certain vortex category. Just purely to make it easier to stream inbounds both to EGWU and EGLL.
But as everyone says you'd need a high speed rail link or similar to EGLL. The residents of Ruislip, Harrow, etc might have something to say about it too!