I feel for you OP, I spend most of my time isolated. I'm on month 11 of my maternity leave, my partner works extra longs and overtime, and is a key worker. I've battled postnatal depression and horrible intrusive thoughts every day.
But moving would be selfish. And not for the right reasons. You are putting your friend at risk. Your friend is putting you at risk.
There are too many people thinking they've got special exemptions that trump the good of every one else.
The more car journeys you do, the more you increase your risk of a road traffic accident. I agree, the risk is slim, but it's a risk that isn't needed when our ICU beds are taken up by people suffering from COVID. If you took up a bed, someone else wouldn't be treated.
If you or your friend catch COVID from being in close proximity to each other when it could have been avoided, that's a doctor having to make a life or death decision with someone else.
It's not about just you. You need to find other ways of managing your mental health. We live in the 21st century, we have phones with cameras and laptops and the internet which allows us to connect to people through text, voice, video and through games. We have forums of like-minded individuals. We have photos. We have letters. We have email.
There are so many ways to stay in touch with other people without being near them. Your journey is not essential and it could cost the lives of others.
Consider it as risky as driving without your seatbelt on. Unlikely to cause any issues but it will be one clusterfuck if something were to happen.