There are plenty of active digs in the area. Mostly weekday evenings.
I'm not too sure if Covid has increased or decreased the amount of digging overall. For me, it's meant more digging as there is nothing else to do and with no pub to go to, we've Bee digging later into the evening without the worry of missing closing time.
Most diggers would welcome some extra help but some digs are more technical than others to get to, so depending on your current fitness, size and experience, some digs may be more or less suitable.
We currently have one dig down a pretty narrow and torturous entrance series and another that's underwater. We could use some help at the former, but that help would need to be able to get themselves into and out of the cave without assistance, preferably carrying a bag of tools.
It would probably be best to get to know some local cavers through a club and if you're keen to dig, it won't take long before someone has roped you into a project.
Quite a drive from Bognor Regis though, so evenings probably aren't ideal.
Portland may be easier for you?