Here's the definitive answer from QCA - You don't have to continue in Year 11:
'Thank you for contacting us on 26th February, regarding mathematics in key stage 4.
'The statutory requirement is that the programme of study for maths at key stage 4 is completely covered. If this done during the course of the GCSE in year 10, then it would be acceptable for these candidates not to need to study maths in year 11.
'The programme of study for maths can be found on the National Curriculum website:
'I hope this information is useful. If you require further assistance, please do not hesitate to contact us. The contact number for the Enquiry Line is 020 7509 5556.'