If you only have British Nationality (you do not also have an EU Nationality) and you did not legally reside in the Netherlands before 31 December 2020, you are now considered to be a ‘third country national’. The rules/regulations for whether you will be allowed to come reside and/or work in the Netherlands have changed. The IND (immigration department) website explains this here.

If you already lived in the Netherlands on 31 December 2020, you had the opportunity to ask for a  Withdrawal Agreement Resident Permit (WARP) until 1 October 2021. If you haven’t done this you may loss any rights under the Withdrawal Act. You can call the IND Brexit Information Line for further information (088) 04 30410.