Is there any income based support available for you- there may be a hardship fund? (At the collage)
If not;
If you are not going to study maths to A Level physics will be too hard (I would consider maths though- it is very well respected)
You could consider doing Biology opposed to geography- it would be good in combination with chemistry
If you did drop the geo and do biology, chemistry and maths you would not need a 4th a level
What do you actualy want to do after sixform?