If you say you will let me know...

[lepetitcolombe]

...why you do not follow up?
Bonjour everyone, I am a bit puzzled because if you say to someone you are ok to meet up and will let me know a date but then ignore this...is this one of this British habit like, how are you but you are not supposed to actually say anything else than very well thank you, what is the correct etichette? Shall I forget about it? Merci! (I am still adapting to the British culture but we love it here and I love Mumsnet )

Well, it depends.
Sometimes people genuinely intend to get back to you, then life gets in the way and it slips their mind.
Other times though, I think people do say it as a social nicety with no real intention of following it up. It is hard to know which is which, especially when you are forming new friendships.

Also when you are new, people might be kerping their options open. So they like you but are not sure yet if you will brcome a friend so it is a sort of 'keep warm' statement.
Does this not happen in France?

Belgium , I suppose it may happen ...as you nicely put it, but had my friends there...I would not do so though..a bit overwhelmed with meeting so many new mums at school, I am so worried I may say the wrong thing and culturally not fit, we are in small town! Thank you!

I think Aderyn is right; it could be either, but please give the other mum the benefit of the doubt and assume it slipped her mind! She probably still has every intention of letting you know a date. Why not send her a text suggesting a couple of dates? There's no reason to say that it's on you to wait!
Oh and welcome to the UK! Hope you're enjoying it here and aren't finding the culture too bemusing!

Personally I try not to say things that I don't mean. I think it's rude to not do as you say. I know everyone is different though.