I've used the Emergency Vets in Streatham a few times, once on Christmas Day! They're obviously more expensive than your standard vets but we've always been happy with the way they've looked after our cats. I seem to remember you have to book in first by phone when they'll also tell you how much the consultation fee will be. The on-call vet is there all night I think, so if you're worried about your dog and want to speak to someone before the morning, just give them a call now. Hope he feels better soon.