One parent has written to me to let me know that you can book a test to bring with you to Prague for the return journey which she found through BA. 
She wrote:
“I thought I’d share this with you in case anyone else asks as it may be helpful. Its a test that you can bring with you to Prague and then you will need to book a supervised online consultation 48 hours before you leave Prague. The results will be ready at the end of the 30 minute consultation and a certificate emailed to you which will allow you to fly back into the uk”