On BA, by the time you've
1. Schlepped out of Paris on the RER (if you're anything like me, always picking the one that stops everywhere),
2. Fought your way through the mess that is CDG T2B,
3. Been delayed due to slots,
4. Endured a massive taxi to the rwy,
5. Had a delhi-belly box thrown at you,
6. Done an aerial tour of North London due to the inevitable hold,
7. Landed on 27R, long taxi to 27L, even longer wait to cross to get to T4,
8. Bussed around the bowels of T4 (CDG always seems to get a remote gate)
9. and finally queued for hours at Immigration if you're not a EU citizen....
you'll wish you'd taken Eurostar! In any case, I think the quickest journey time is now down to 2h40 due to the high speed link being half-open.