Sorry to hear this,
Weller ! It's never a nice time to get ill, but the first day of a new job is really annoying.
Don't worry about UC, just think about it as you would any new start. The point is, you can't assume it's not COVID, however unlikely it seems. Better safe than sorry, and the DWP won't penalise you for sticking to government guidelines anyway. And if the new employer is decent, they should hopefully understand and let you start a week later. Your UC payments will carry on as normal until your wages come in, then they adjust. It's not as if you're not turning up to work, just explaining you're not well enough to come in/self isolating because of symptoms of what could be COVID. If you haven't already told UC about the new job, I wouldn't until you've started anyway, as temp jobs can fall through at the last minute.
I hope you feel better soon.