I was called up recently too.
I am a full time carer and wrote explaining this and was sent a reply within the week saying I was exempt for life. It's a shame really because I would really have liked to have a go but it's just not practical for me.
You need to ensure that you return all the paper filled in and send a covering letter asking for an exemption. They may not feel your reason good enough, and they will check up on claims of being a carer etc.