Possible to return to a PhD years later (at the same or a different uni?)
Whether your university (or another) will allow you to pursue your course in a particular fashion is something you will have to ask them.

The key question to consider though is how your research will be impacted by the delay. I guess ancient history may move slowly enough that it's unlikely someone else will do the research while you are waiting. But how well will you be researching while doing something else/after you haven't thought about your topic for a long time?

A more common approach in your situation would be to transfer to a shorter qualification (some sort of masters) that you can complete reasonably easily using the work you have already done.