I did A level modern History (1870-1945) after doing the GCSE.
However, i `came out of retirement` to do A level Ancient History without first doing GCSE and having next to no knowledge of the period, and i managed an A.
A level is an entirely different kettle of fish to GCSE, if you are well read and have a passion for the subject, i really do not see why not.
My advice is read, read, read!
Maybe find out the syllabus and do background reading on the period and find out what text books you will use.
This was how i studied for Ancient History and now Classical Civilisation doing these subjects entirely on my own, use more than one author and get a `feel` for the period and a narrative in your head.
At the end of the day the only thing you can do to persuade them is emphasise how much you want to do it and plead for an exemption.
Does remind me when i was in Year 13, my Economics teacher stated that he felt people without at least a grade `c` and preferably a `b` in GCSE maths were not suitable for A level Economics.
I had great delight in informing him that i only had a `d`: by that time i had already scored several A`s in mock exams.