I am a full time PHD student on Tier 1 general visa.

Can I switch on to part time?

Is it possible?
Or can I work full time as well as be a full time student?

You will need to check with your uni.

In my uni we are allowed to work up to 20 hours per week while in a full-time funded PhD.

A student at my university wanted to change to part time and is on a Tier 4 visa. The answer was no.