accepted a place but seen another course, help!!!!

Hey, worried undergrad here.

I've accepted a place to study English and History at LB university, after deciding to decline my offers at Leeds, and York.

I thought I was happy with the course, but I've just found out I would have been eligible for special consideration at Leeds and York (the access schemes) and I also took another look at the courses.

I've found an ancient history course at Leeds that I didn't see when I was applying. And a similar course at York. I didn't see either of them on the course list when I applied, which I'm putting down to me not being that great with technology. (surprising for a millennial, I know.)

What can I do?

Would I be able to try and apply through clearing, even though I've already accepted an offer somewhere else?
Is it possible, and what would be my chances of there being a clearing place?

Has anybody else done similar?
Help!