As above most PhDs are entirely research in the UK as I understand (there's a few MRes+PhD schemes or similar but most are just standard research degrees). That said for example in most STEM fields that means you're going to be 9-5 in the lab or office doing data work. For humanities and social science fields I would expect a good chunk of time would be spent on campus using library resources and other collections depending on your specific area.
I imagine in all fields you'd also meet with your supervisor regularly, probably attend some seminars/journal clubs and similar regularly, your supervisor might also recommend you attend certain taught classes to audit them to fill gaps in knowledge, etc. If you also do marking/demonstrating/teaching work in the course of your PhD you'd also likely be doing this on campus.
Also worth noting in general Cambridge requires students to live within I believe for postgrads 25 miles of Great St Marys in the city centre, so you'd still need to be local to Cambridge unless given dispensation to live away (which would probably be more for e.g. fieldwork, time spent at a partner institution or industrial secondment etc. They may not give it if you just wanted to live away from the uni generally and conduct everything via teams or similar).
Ultimately though I gather a lot of it really comes down to what your specific research project is and how you negotiate the work you do with your PhD supervisor. If your project uses primarily or wholly digital material and your supervisor doesn't feel you need to be coming in regularly you may not need to go to campus that much. There may however be an expectation you are visible and present broadly in working hours as part of your research group or similar for example though.
By way of anecdote, I have a friend doing a humanities PhD (full time), and he doesn't have to go into the uni every day. However as he does teaching he does need to go in several days a week for that, and also to go up and attend specific meetings, or to get specific things from the library and/or return library resources he's no longer using. He also likes to catch up with his supervisor regularly so ends up going in most days as a result anyway even if he doesn't NEED to (but has the option to work from home if there's a delivery or a contractor coming or something). By contrast, a friend of mine who did a STEM PhD had to go into his office every single day of the week because he was doing stuff in the labs most days, or teaching/demonstrating, or attending journal clubs/supervisor meetings, etc.