@MrKlaw, our LA is providing testing for school returnees, teachers etc at least - it gives the following instrucrions:
How do I book a test if I do not have symptoms?
Go to the government website www.gov.uk/get-coronavirus-test where you will be asked a series of questions:
Choose NO to essential worker
Choose NO to having symptoms, and
Choose NO to being part of a trial or pilot
Then select "My local council or health protection team has asked me (or someone I live with) to get a test, even though I do not have symptoms.”
You can book your whole family through one form by choosing 'add family members'.
Selecting these options will allow you to book to access a COVID-19 test.
Might be worth looking at what your own LA website says (or asking friends with secondary age DC - if you haven't
/ their school hasn't said anything yet re going back) whether their school(s) have issued any advice?