Vet med Watch
You won't have to rely on Clearing if you apply through UCAS. You can put down your achieved grades in addition to your predicted grades for maths and physics. Any offer you get will appear as 'conditional' until results day 2019, where it will become successful or unsuccessful based on the grades you achieve. For the record though, in the past couple of years there actually have been a few vet places in clearing. Regular medicine too
As for 'optional places', you can apply for deferred entry (i.e. for 2020) which would allow you to take a gap year, but it is essential that you meet the grade requirements in the year that you apply. You don't get an extra year to get the grades, if you fail to meet your offer conditions on results day 2019 then your offer will be withdrawn and you'll have to apply again.
Hope that cleared a few things up, best of luck with your application.