Just had a skim through the
https://www.gov.uk/government/public...ance-bill-2018
which basically intends to provide some sort of oversight of the UK environmental behaviour post Brexit (specifically). There's a lot of the usual bureaucratic guff in there, but as far as can see the body they plan to set up (Office for Environmental Protection (OEP)) won't really have much in the way of teeth, and essentially no powers over the government nor parliaments and the devolved bodies. In other words, while up to now the EU has had some powers to take the UK to task for environmental issues, post Brexit there will no one in a position to challenge the UK government etc.
I rather suspect the tsunami of new legislation and overseeing bodies set up by the government and parliament to cover all the aspects of regulation will tend to result in more unchallengeable power in Westminster.
"Who polices the police?"
It's sovereignty I guess. Just keep an eye on them, that's all, you know what they're like.