I was treated at Mount Vernon 10 months ago, and had a 40 min drive each way.
I would say, if you're fit, you will manage it without any problem.
If you are not fit, you might become quite fatigued towards the end of your treatment, but I don't think it was enough to prevent anyone driving. Sometimes they will arrange transport, but I'm not sure what the rules are for that.
It was a really nice place to be treated last summer. Everyone really friendly, and they look after your health during and after treatment very well, much better than many people report from other places. I really enjoyed going in each day, as did most of the prostate cancer patients. Sadly, some of this is bound to be different in the light of COVID-19, probably no sitting around talking with other patients and staff. They have bunkers for 12 LINACS (the radiotherapy treatment machines), although only 9 were in use when I was there (the spare bunkers are used for commissioning/decommissioning LINACS). The LINACS are all modern image guided volumetric modulated arc radiotherapy (IG VMAT).
I run the prostate cancer support group there. We can't meet at the moment due to COVID-19, but I'm inviting the members to larger groups which are running Zoom meetings and still providing one-to-one support. I'll PM you about joining.