I have had success on this side of the Atlantic by using London & UK restaurant booking, reviews, recommendations - toptable.co.uk
They often have last minute cancellations at some of the hottest restaurants in the country.
Mind you, I recently (couple weeks ago) wanted to book a meal at Martin Wishart's restaurant in Edinburgh (he's a Michelin starred chef)... I was told, in a rather sniffy manner, that giving them 3 months notice wasn't enough.... they are now taking bookings for 4 months hence... Ho hum, the pretentiousness of restrateurs is becoming amost too much to bear!
:roll: Needless to say, I booked elsewhere. Who knows if one would still be alive, 4 months from now?!