The site in question is charts.dft.gov.uk (VERY NSFW). It resolves to the CNAME charts.dft.gov.uk.s3-website-eu-west-1.amazonaws.com, which is quite clearly hosting a porn site of some kind.

I suppose there's a few possible explanations here: (1) the original site was hosted on S3, and at some point the bucket was dropped and someone else picked it up, (2) it was originally hosted on S3 and the bucket got hacked, (3) someone with access to the DNS has decided to go rogue and point it at a somewhat-legit-looking but fake domain. If there are historical DNS records floating around it might help to narrow down what happened here.