You are not automatically insured when using your own car for a u3a activity, you will need to check this with your own car insurer. See also: ABI: Volunteer Driving, Motor Insurance
Yes. But it is recommended that this is done as an informal arrangement between members. The guidance is that car sharing should be an informal arrangement between drivers and passengers. Anyone acting on behalf of the L&M u3a should have no involvement either regarding who goes with whom or how much the fuel contribution should be. They may only suggest or recommend.