Hopefully we can get the Naturalisation Cert & Thai passport back earlier, in time for the trip. I'll call during working hours tomorrow & speak to the passport office directly rather than their evening call centre to try and arrange their return.
The question remains though, would wifey be breaking some sort of rule by leaving the UK while her application for a passport was under consideration?
That's what the call centre told me. I find it hard to believe.
However, I remember a colleague falling foul of some rule by posting his 2nd passport to a visa agency while he was abroad using his 1st passport. Customs recognised the address, seized it and he had to go cap in hand to get it back. Cost him a fine as well. It's clear that there are some obscure rules out there.