OMG, I really need a second chance of edit!
On second reading of fares, it IS NOT cheaper to travel to Euston on Anytime Day return ticket and then take the tube on contactless, as the Euston fare costs 50p more on Anytime Day return than to Kensal Green, but of course other combination of fares exist subjected to time restrictions.
https://www.brfares.com/!fares?orig=LBZ&dest=EUS&period=20231001
https://www.brfares.com/!fares?orig=LBZ&dest=KNL&period=20231001