The 10 years will almost be as much as it will take any new runway to be built.
It won't be possible to have a new aiport in the Thames estuary up and working by 2022, it would be 15 years to phase 1, 20 to phase 2. Heathrow runway 3 was already approved remember, so it needs dusting off and re-approved. Realistically that is do-able in under ten years, particularly now the rules have been streamlined.
I understand the idea is that LHR runway 3 will be sold as a "necessary evil" until the new airport is built. What you cannot say is that we can do nothing for the decade and a half until Fantasy Island may open. That's understood to be a stupid idea by enough people that ways are being looked at to U-turn without U-turning.