Can be really fast. SGN is rather small - not much walking involved. If you're among the first off you won't have to wait at immigration. The worst I encountered was arriving just after an AirFrance flight - think it took 25mins. Then down the stairs to collect baggage - usually they're pretty quick.

Overall, its the fastest airport I use. With hand luggage only, I've been off the plane, through immigration and customs and into a taxi in under 10 minutes. I would say 25 minutes is typical if you have non-priority luggage. Allowing 10 minutes to get off the plane, then if its on time I'd guess you could be getting a taxi at 6.35PM. 6.45PM if you want to be a little more prudent.

British passport is nothing out of the ordinary. From memory you're processed either as ASEAN or non-ASEAN. Just make sure you don't join a queue for a booth serving locals/ASEAN only.

You probably know this - you need a visa.