If you take payment of any kind you're likely to be classified as driving for "hire & reward", which you definitely wouldn't be insured for on a regular Social, Domestic & Pleasure (+ Commuting) policy.
The chances of being caught doing that are perhaps remote, but stranger things have happened. Plenty of reports on Pepipoo about people doing their mates a favour transporting takeaways etc and getting done for no insurance because they don't have business cover.