On 5 June 2018 the Government announced its proposals to back additional runway capacity in the south-east of England - specifically a new north-west runway at Heathrow. This proposal is subject to Parliamentary approval which, if given, could mean an operational third Heathrow runway by 2026.
The statement by the Secretary of State for Transport to Parliament can be found at: https://www.gov.uk/government/speeches/proposed-heathrow-expansion
The Government press release can be found at: https://www.gov.uk/government/news/government-agrees-final-proposal-for-heathrow-expansion
A 1 page overview of why the Government supports a new north-west runway at Heathrow can be found at: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/713755/why-we-prefer-a-new-runway-at-heathrow.pdf