EU (Withdrawal) Bill - Greener UK & Friends of the Earth

The Government has made repeatedly clear that all EU legislation will be converted into UK law at the point of the UK's departure from the EU, and Parliament has the power to modify retained EU law in the future.
The environmental principles that you mention have had an important influence on EU environmental legislation which is itself being transferred into UK law. The EU (Withdrawal) Bill will ensure that, wherever practical, the same rules apply after we leave as they did before.
Although the jurisdiction of the European Court of Justice will end, the Government will ensure that strong and effective governance arrangements are in place after the UK's exit from the EU. No decisions have yet been taken on the UK's future relationship with EU agencies including with the European Environment Agency. This is a matter for the negotiations.

In my view, when we leave the EU we can develop new institutions which will be able to hold us to even higher standards. It is our intention to establish a new world-leading body, independent of government and able to speak its mind freely. It will have clear legal authority and will champion environmental standards, always rooted in rigorous scientific evidence. The Government will consult widely on the precise functions, remit and powers of the new body but I am in no doubt that it must have real bite.
A formal consultation will be launched early in 2018 and we must decide what functions and powers the new body has to enforce laws. The Government will consult with as many people and organisations as possible - from businesses, the farming sector, civil society, the devolved administrations and elsewhere.
Nothing can be more vital than the future of our environment and the natural world. We are their custodians and we must safeguard their future if our ambition for a Green Brexit is to become a reality. We have the chance to set the gold standard for environmental science and become a home to centres of environmental excellence and I am optimistic that we will succeed.