That's misrepresenting things. He's said british planes have been in the air protecting british air bases and personnel. They UK has not been involved in the strikes on Iran.

Starmer's rhetoric however is aggressive, blaming this entirely on Iran when the US and Israel have decided to attack Iran. I'm no fan of Iran's regime, but I'm not a fan of countries unilaterally attacking other countries, and Starmer is being a hypocrite given he usually talks so much about "international law" but is not condemning Israel and US aggression.