I’m currently in a gap year and applying with A levels already done. You need to find the number for admissions for each university and call them, department heads normally won’t know anything its the admissions team that will.
In terms of deferred vs applying after. If you apply with A levels already done you either get rejected normally or receive an unconditional offer. This means unless you reject the offer the place is guaranteed. In contrast some unis don’t like deferring places without good reason eg. family circumstance, medical issue etc. and may hesitate to give you an offer so unless you have a strong reason i would’ve recommend it. A normal gap year is generally more flexible as long as you can say it wasn’t spent doing nothing eg. work experience, travelling, studying etc.