Hypothetical scenario here.

Say I had 1.5 years of funding left on a full-time EPSRC funded Studentship, and I was offered a full-time industry position (not necessarily related to the PhD). Could I then:

Switch my funding from full-time to part-time, whilst also working in a full-time industry position. Assuming both that the job would be ok with this arrangement, as would EPSRC.

Does anyone have any experience with an EPSRC Studentship of doing exactly this?

I do not think it is possible. The reason for funding will exist no more. If you say, I have for private reasons to switch from full time to part time, the funding body might accept. But if you get a full salary, you are not "blocked" from gaining money and you will be like other part time students. You might try to get exemption from tuition fees though.

Your supervisors will not be happy but there is very little they can do about it. The funding and the PhD enrollment are separate, so your PhD position is not dependent on EPRSC funding. Going part time will probably stop the EPRSC funding but they can't kick you from the PhD unless you fail something.

Though, I am assuming that you are an experimental PhD student and if you go into full-time employment will you have the time to do the necessary experiments. It will be hard to manage lab commitments and work commitments which is ambitious.