I know its a bit late but....
How did your school cash in your grades?
It is possible to for your school to override Edexcel's Cash in Code if it is in your best interest (within 7days of getting Edexcels result sheet).
In your case you would have got grade A maths and grade D/E for F.Maths, and you would have got into Oxford (A maths, A econ, A phy). (This is assuming that your school was wise enough not to cash in lower sixth for AS)
I thought this might be useful for other people in your situation

I did talk about this with my school, Sadly I achived fairly low B's in most
of my maths modules, so even combining my 6 highest I would'nt have got an A
in maths. I scored a B in AS maths and so already had some catching up to
do. I did well in P3 but did rather poorly in M2 which meant there was no
way I could achive an A in maths.

If i was really desperate to go to Oxford I would have probably retaken the
modules and applied this year, but I didn't really want to do that.

You seem to know quite a bit about cashing in grades and things, would you
be interested in writing a piece about it for my site?

Cheers

Tom