However, there's a decent analysis here of why the British entry didn't do well - by bad luck it was too similar to other slightly better entries. "if your vibe is hyper-catchy, female-fronted pop, you were spoiled for choice". That can happen. Nobody seems to think it was a duff song or singer.
There is a bit of a clue:
"on the night, she brought out the big guns. Mae sang better than she'd done all week."
Except that the jury vote on the Friday night performance, not "on the night".
Eurovision is not a "song contest" though: that implies songs literally battling with each other. The song was ok, but not a patch on Spaceman. People vote for what they like, and plainly they didn't like "I Wrote a Song" enough.