I would not book 2 single tickets, too expensive.
This is also trueMy wife has spoken to the Thai Embassy in BKK and they have said that there shouldn't be a problem changing the name on the passport and visa once over there...
The problem you have is that neither the airline or the embassy are worried about the others rules.
Why change the surname in the passport? why not just renew it in her maiden name and there is no hassle with her using her new passport as it will match the airline ticket.
Save the name change until she gets her British passport.