If they are fully vaccinated then they should have no issue. From the UK government site
Who needs to take a test
You must take a COVID-19 test before you travel to England if you either:
do not qualify as fully vaccinated for travel to England
have been in a country or territory on the red list in the 10 days before you will arrive in England
You do not have to take a COVID-19 test before you travel to England if you qualify as fully vaccinated and you have not been in a red list country.
This also seems straight forward for their return to the Netherlands
Fully vaccinated travellers arriving in the Netherlands from the UK via aeroplane, ferry, passenger car, train or bus must show one of the following:
a negative PCR test result (taken no more than 48 hours before departure);
a negative antigen test result (taken no more than 24 hours before departure).
Fully vaccinated travellers are exempt from quarantine on arrival in the Netherlands.
UK nationals who are legally resident in the Netherlands
UK nationals who are legally resident in the Netherlands are allowed to re-enter the country regardless of their vaccination status, but will need to show one of the following:
a residency permit
a certificate of application
They may be subject to questioning by Dutch border authorities when they arrive in the Netherlands.
This measure does not apply to those travelling to the Netherlands from a country that is on the list of safe countries/regions, published by the Dutch government.
And of course the passenger locator form requirements that can now be a lateral flow test
Fully vaccinated travellers are required to:
book and pay for a day 2 COVID-19 test – to be taken after arrival in England
complete a passenger locator form – any time in the 48 hours before arrival in England
take a COVID-19 test on or before day 2 after arrival in England
Caveat that this may all change at any time.
John