I hate to recommend a successful chain, but this food is fantastic, the atmosphere is loud and fun, and you will remember it. High chairs happily provided, which is sometimes unique in hip places to eat.
For locations before you go, look at www.wagamama.com. The one listed is around the corner from the British Museum.
For special prices, sign up in advance for their email list, or look for 2-for-1 coupons, often available in Time Out. The green tea is free.
Wish I could go there right now.