Just booked the Focus in for it's 2 year service and automatically chose the nearest dealer (also the place we bought it from)... Afterwards (on a tip from a friend) I also had a look at prices through ford.co.uk's on-line service booking...
My local dealer wants £120 - most around here (Berkshire) want £140 and up the road in Buckinghamshire they want £180... Now £20 isn't that exciting (but you'd still be saving even after the added petrol costs) but for £60 I reckon for most people it's well worth the extra distance.
Just thought it was worth pointing out as I thought there wouldn't be much difference in price between official dealers (especially ones in a 20 mile radius!)