Thanks guys, i'll keep checking! I didn't realise there may be spots after midnight.
If I don't get one before 22nd Jan, would it be a problem?
I'm not sure how strict they are with the deadline... earlier this year, some people had to wait 5-6 months to give biometrics because of Covid delays/centres being shut, so I don't know if it will matter if you can't get one by 22nd Jan.
Having said that, back in Aug/Sept/Oct, a lot of people ended up having to pay for a £110 appointment anyway, because there were just never any free appointments available when they checked, so you may find that you just have to pay for the £110 appointment if you don't have any luck with booking a free one.
The Visa expires on 15th Feb.
As long as you have applied online before 15th Feb, it doesn't matter whether or not you attend biometrics before your visa expires. If your visa expires before your appointment/before you get the new visa, your immigration status will be automatically extended under Section 3C Leave while your visa is processing.