What are my options?

What do you want to study? Do you have any idea?

I'm definitely thinking of Maths now

To study Maths you need Further Maths so you would need to take that at AS level this year. (Most universities like it as a full A level but some are happy with just the AS). A maths-related degree may be more appropriate depending on your interests... but obviously it's your choice.

When I picked up my results my head of sixth form told me that she would be happy to sandwich in further maths if I wanted, so do you think that's a good idea? Or should I stick to the AS and only drop 1 subject, rather than dropping 2 and doing the full a level... Worried about the workload

You're clearly very talented at Maths, so if you worked really hard you could probably do it. But it would be a huge amount of work, so it's up to you - if you think you could cope, go for it. You can always drop down to just the AS if the work gets too much.