The official line is that as a EU citizen you can enter, full stop. You will need to have had a test beforehand (done 48 hours before entering Italy) in order to board the flight (or a vaccination, but that doesn't apply in your case). Then, because it's a UK arrival, you'd need to self isolate and do another test.

The two exceptions that are quoted are a) up to 120 hours if you're entering Italy for proven needs of work, health or utmost emergency or - as you said - because you're transiting in a private vehicle.

Personally I'd say that you'll be allowed on the flight from the UK, provided you've tested negative for Covid. Once arrived, I think there are two choices: if the airport (I'm thinking MXP, or FCO) have a transit facility, use that one especially if you're hand-baggage only or have managed to interline the bags. If not, then I'd simply go through passport control and be honest about it; you'll also need to have filled in the PLF to state you're not stopping in Italy.

My advice, if I may, would be not to do a self-connection in these times. I don't know why you don't want to fly directly to Poland (I think LOT and Wizzair, plus perhaps Ryanair, are flying there from the UK) but it just adds risk.