Re: Welsh female name pronounced "COO-pee"
I suspect it probably is Toby, in a really mangled screen accent. I'm Welsh and a long-time collector of Welsh names, and I've never come across any name sounding like coo-pee, which as Llewella said would be 'cwpi' in Welsh. I looked it up too, and found nothing useful. Obviously without having seen and heard the film I can't be sure, but 1940s screen representations of British regional accents are not known for their accuracy.
The book by Humphrey Slater that the screenplay for the film was based on is available on Amazon (although outrageously overpriced), so it's probably possible to order it via a library and find out what the character's name was meant to be.