I started my PhD in the humanities in October - if I choose not to do my upgrade and to do an MPhil so I can finish a year early, would I be able to just do the 2 years of a PhD at a later date, or would I need to start the whole process from scratch, even if it is the same project?

If you have claimed the MPhil as a qualification in itself then to get a PhD you would have to start from scratch all over again.

I'm pretty sure that you can't 'top up' the Mphil into a PhD at a later date, and you certainly can't keep the Mphil and do a quick PhD in two years to have both qualifications.

I'd second what XJR has said

How long a break do you want to take? If it's a few months your uni might be able to arrange a leave of absence