Need advice if this is worth it.

Ask yourself why you only got an E and whether you can improve, want to or need to.
Isnt there another subject you can do?
Talk to your maths teacher.

Your choice whether you carru on, but if it were me I would take soemthing else, unless I really needed maths, in which case I would retake AS.
You could do A2 privately if you have the money, but next year is the final chance for legacy resits.

The E was clearly due to a lack of hard work on my part as coasting through the year cramming in the last week does not work like gcse's as I found out which is an attitude I know I must change this year.

And I was considering dropping it but the school would only allow me to do an AS and ideally want 3 A levels to take forward.

You said next year is the final year so if I cash in my AS grades and A2 grades that I can do privately would that give me the full maths A level by cashing it all in or is it not as straight forward as that?

You could do AS and A2 all in 2017.
Its whether you need maths and want to or it wouldnt be easier to do another subject.