London isn't cheaper than the Valley at all. Average house prices in central London are £5000 a square foot. Slightly further out they're around £1500 a square foot.
Compare that to the valley, where in the south bay they're around $1500 a square foot in central SF, and $1000 a square foot in the south bay. (note, different currencies, so the delta is bigger than it looks). London is both more expensive than the bay area, and they'll pay you less.
As far as urban sprawl goes... the Bay Area is far less sprawled than London. Even if you are in the centre of the largest spread of sprawl in the bay area (probably somewhere slightly west of the centre of San Jose), you're at most 20 minutes from open country side (by car). In London, in the centre, you're roughly 2 hours away, such is the size of the sprawl.
Even if you measure sprawl by the time it takes to commute, and assume a worst case in the bay area (living in Oakland, working at Apple is probably about the worst) - that's a 1 hour commute. That's a pretty short commute by London standards. More so, the commute in the bay area will have been done entirely on nice (not crowded) coaches, rather than over-crowded, not-air-conditioned trains and underground services.
Are there really people who love the urban sprawl that covers the entirety of Greater London?