Polpo doesn't take reservations and is nowhere near the same level as L'Arpege or L'Ambroisie (both 3 star Michelin restaurants representing some of the best cuisine in Paris).
I think the closest you will get to this fine level of dining in London is Ledbury. This is a 2 star but Brett Graham is one of the most talented chefs right now in London. Dinner by Heston Blumenthal should also be a good fit with a very different feel and cuisine than Ledbury. Gordon Ramsay's Royal Hospital Road is a 3 star but Ledbury is more consistent and inventive (read: better). River Cafe is a classic and solid.
If you can make it out to the Fat Duck then that would be an experience. For something really unique check out The Sportsman (not in London though): http://www.thesportsmanseasalter.co.uk/
For restaurant recs you really should be consulting chowhound.com for more reliable advice.
With regard to Paris - be aware that L'Ambroisie is notorious for favoring "friends of the house" and regular patrons with new diners sometimes receiving underwhelming service. Pierre Gagnaire is another fantastic restaurant with impeccable service and food (much more modern than the others mentioned). Le Cinq at the Four Seasons George V is the full package with ambiance, service and cuisine all being top of the line. L'Arpege has very simple preparations of superlative ingredients and the room/service are a touch more casual/homely than the above. That being said, one can have a meal there that will linger in the mind and often surpass anything else in Paris at the 3 star level. Guy Savoy will be more of a party atmosphere with the staff treating you like family. Food is not the best in Paris but the experience is a memorable one. Enjoy your trip.