The European Union (Withdrawal) Act 2018 ("EUWA") establishes the main underpinning legal framework for the UK’s departure from the European Union. Whilst the UK famously does not have a written constitution, EUWA like the European Communities Act before it, has great significance for the UK’s future constitutional arrangements.
The key elements of EUWA are summarised here and you can find a range of other resources on Mills & Reeve's Brexit Hub.