biology teacher employability with this...

You're going to have to teach GCSE Physics/Chemistry anyway in all likelihood, so you may as well do the Subject Enhancement. Better to spend a few weeks giving you some confidence than spend years teaching unconfidently.

Yes, Bio is in less demand, that doesn't mean Biologists can't get jobs (any more so than History, English or Art teachers). There are currently 48 jobs being advertised on TES for secondary science or biology teachers. If you are worried about it so much that you are letting it stress you out around 4 YEARS before you're going to have to start applying for teaching jobs (assuming you are a yr13 now, doing a bio BSc next year and not taking a gap year) you seriously need to chill out a little.

What's your backup

Oh right, well thanks for that piece of advise
So, do you think i would struggle to get onto a PGCE with biology as the only science a level?

I don't think you would be hindranced by doing a PGCE in Biology with a Biology degree, but it could help your future employment if you can demonstrate that you have (some) knowledge of the other sciences through modules that you've done at University.