If it's the exam I'm thinking of, diverting to avoid it puts you in the middle of nowhere, with no decent features either to recover to track or to know when you've flown clear. But the planned track through the MATZ actually keeps you clear of the embedded ATZ.... You can't go over, because the cloudbase prevents that - and you can't go underneath because the base of the section involved is ground level.
No doubt the intended answer is to divert around; however, if I was in the situation I would fly the planned track after asking London FIS to confirm the frequency and availability of the MATZ crossing service and asking them to relay my intention to the MATZ controller.