Yeah i got it now, so if i wanna stay in LON that's a good choice
If booking a RT, and with advance purchase (7 days) and minimum stay (Saturday night) requirements being met, it's oftentimes not worth redeeming miles for Economy travel. Especially to a destination / with an airline that charges high taxes or fees.
Mileage shines for OW travel, or if you don't have advance purchase or minimum stay. And of course for premium (Business / First) travel.