We are planning a trip to Europe (France Switzerland & Spain) and I have just noticed that my wifes UK passport has expired. She also has a valid NZ and Thai passport. In order to avoid the hassle and cost of getting a new UK passport or getting visa for her Thai passport can she use her NZ passport whilst I use my UK one for our trip

Yes, she can use her NZ passport to enter the Schengen area as a tourist for up to 90 days.

See "Visa-free access for New Zealand visitors to Europe" from safetravel.govt.nz

She will have to leave the Uk again on her NZ passport in order to avoid overstay.

Best to sort her new UK passport with the fast track 1 day service IMO

She will have to leave the Uk again on her NZ passport in order to avoid overstay.

Best to sort her new UK passport with the fast track 1 day service IMO

I suspect she can re-enter the UK again on her expired UK-passport without any problem.

She can't travel on it, but returning to the UK on an expired passport is no problem.

 

I suspect she can re-enter the UK again on her expired UK-passport without any problem.

She can't travel on it, but returning to the UK on an expired passport is no problem.

I guess she would need to get a new passport in the UK during her stay there though, as she would need a valid passport to exit the country and neither her New Zealand nor her Thai passport would have an entry stamp.

 

I suspect she can re-enter the UK again on her expired UK-passport without any problem.

She can't travel on it, but returning to the UK on an expired passport is no problem.

I guess she would need to get a new passport in the UK during her stay there though, as she would need a valid passport to exit the country and neither her New Zealand nor her Thai passport would have an entry stamp.

Stamps don't matter, so long as she has evidence that she can enter Thailand legally (via the Thai passport) she'll be fine. Having said that, if you are in the UK, getting a new passport there will probably be simplest.

For Switzerland, France and Spain, the NZ passport works just fine. They'll barely take a second look if she flashes it.

be prepared for a long wait if you are applying in Hong Kong Via Mail.