Why is the possible british invasion of holland so importaint to the residents of the annexe?

This event would have a huge impact on those hiding in the annex.

They are hiding in Holland, so a British invasion would directly impact them. While they'd welcome the British removing the Germans, they would have to contend with Germany defending their territory first.

To that end, the Germans treated to flood Holland. If so, the members of the annex would have to decide whether to stay or flee. Either one could be deadly. The Germans might also forcibly evacuate Holland. If that were the case, they would again have to decide whether to continue to hide in an abandoned city or leave and risk being caught by the Germans. 

Ultimately, they decide that it would be best to remain in hiding.