Hi mate, here's a page from Joe friels 4th edition.
If you aren't confident in the wet and aren't riding particularly good tires keep the bike as upright as you can.
And plan for the unexpected, ie oil diesel, leaves+ can you stop or reroute before you smack that fallen branch without causing harm to the rest of the group you're with.
I wouldnt change tire pressures or try anything new on the day that you aren't well versed with. Other than going slower.
Also bear in mind rain is normally accompanied with wind so watch cross winds. Bloody windy today in Gloucester. Pretty terrible. Nearly got a lift rather.