You would expect a Frenchwoman of the time to be Catholic; often when you have a 'mixed' marriage you see people marrying in the register office (neutral ground). A marriage in a Catholic church would be a big flag towards her being French as opposed to French-born-British. Another thing to check - did Caroline sign her name or give her mark?
Okay, a slightly off-beat possiblity:
in 1871, there is a David
Foulkes living in Wales:
RG10/5739 folio 6 pages 7/8
( Llangystenyn, Caernarvornshire)
David Foulkes, 39, market garden b. Denbigh Llanchian,
Ursule A. F., wife, 34, b. France
Arthur D. D. son, 12, b. France (British Subject)
Adolfe A, son, 6, b. Italy (British Subject)
Elizabeth, dau, 3, b. France (British Subject)
Albert O, 2, b. Llangystenyn
Alfred W, 1mo, b. Llangystenyn
and not too far away at Llanbeblig near Caernarvon (which was an important port at the time) there is a Caroline Foulkes, 24, b. France British Subject, a servant (age is off but not impossibly far off and as it was her employer giving the info it might be faulty - or she might have lied and said she was older than she was to gain employment).
See:
http://gw4.geneanet.org/index.php3?b=pierfit&lang=en;p=david;n=foulkes (you will need a geneanet account)
Shows that David Foulkes was previously married to an Elizabeth Williams who (if you look at the entry for his marriage) died at Nonant in Calvados in 1857, and he then married Ursule. So Caroline could have been David and Elizabeth's daughter.
He seems to have been working for this company, which gives you the railway connection:
http://en.wikipedia.org/wiki/Chemins_de_Fer_de_l%27OuestFrom that link one of the sons went back to France as well.
I'll recheck the consular records for "Foulkes" later.