Thanks all for sharing your expertise.

Singles appear more expensive than returns, so there's no advantage to me changing my ticket.

I'm not concerned about the risk of being billed for not using my return ticket, as they would need to take me to court to get the money and from all I've read, they would lose. I'll be flying back to the UK with a different airline, so they can't identify that I'm doing this.

If queried, I'll get my partner to say something like "he's traveling to the airport separately" (which will be true!). As long as the airline can't stop them from flying, then I'm ok.