(2-part post):
I've got a 'pet hate' about AB's Travel section. It's all those people who tell us where they want to get to but forget to say where they're starting from ;-)
When you say that there are outbound flights on the 22nd, but none back on the 23rd, you might be referring to either East Midlands airport or Bournemouth airport (which means that you could be almost anywhere in the UK).
In the absence of any other information, I'll assume that you're within reach of London. In which case, your statement that there are no return flights on 23rd isn't strictly true. There are certainly no
cheap flights but Ryanair has a flight to Stansted and BA has two flights to Gatwick.
However, the cheapest way to do things might be as follows:
Fly out from Stansted by whatever route you were originally considering. (Ryanair have a flight at 1730. It arrives at 1950. The fare is nominally �0.01. With taxes and charges it comes to �25.21)
http://www.ryanair.com/site/EN/
On Sunday, take the 0849 train from Nantes to St Pierre des Corps. It arrives at 1033 and connects with the 1041 shuttle service to Tours (arrives 1046). The fare is �19.00. Details are here:
http://www.sncf.co.uk/