The Aviation Safety (Amendment etc.) (EU Exit) Regulations 2019 – Schedule 1 amending the Air Navigation Order 2016.
You’ll see there the various changes, eg:
for “an EASA” substitute “a Part-21”
for “non-EASA” substitute “non-Part-21”
The regulation is made under the EUWA but it will not be in force until IP completion day (31 Dec 2020 at 2300z, but most likely to be pushed into 2021). Until then things remain as they were prior to the withdrawal from EU.