We regularly use Dalat in Northenden, Manchester, for a takeaway treat. The food is very reasonably priced at around £6 - £7 per dish, and having spent time in Thailand mixing it with the locals, I would say it's the closest thing to authentic Thai cuisine I've come across in the UK.
We have had on occasion minor mix-ups with the orders, but these are usually just things like bringing a Coca-Cola rather than a Diet Cola. The food is alway warm, fresh and in good sized portions - and we are always amazed when we order the soup-style dishes at how they can be unpacked to look exactly like they would in a restaurant.
We haven't been to the actual restaurant, but I do travel past it everyday, and it looks a simple affair - classically Thai some might say.