The GDPR regulation is implemented into British law as the Data Protection Act 2018 [1]. It must remain until the end of the year. If there's "no deal" then, next year, the UK Parliament may repeal or modify it as it wishes.
However, there were data protection acts in 1998 and 1984, and it seems unlikely the general principle of access to personal data -- which was there before GDPR -- would be removed.
However, there were data protection acts in 1998 and 1984, and it seems unlikely the general principle of access to personal data -- which was there before GDPR -- would be removed.
[1] https://en.wikipedia.org/wiki/Data_Protection_Act_2018