Possibly the decisive stage of the 2011 Vuelta, British riders lie 1st and 2nd on GC, and it won't be shown live on either ITV4 or British Eurosport.
Since you ask, this afternoon ITV4 has seven hours' worth of British Touring Car Championships, while Eurosport has US Open tennis on one channel and Moto GP on the other.
I'm assuming there won't be any online coverage from Eurosport, but Ned Boulting has tweeted to say that the ITV4 player WILL be showing a live stream, without adverts,starts at 1445, link here: http://www.itv.com/lavuelta/2011/live/.
He added that the channel has a longstanding contract with Touring Cars, which adjusts its calendar so it doesn't conflict with ITV4's TDF coverage.
Frustrating, but hopefully you can get near a computer.