You can redeem your Kelloggs 2 for 1 voucher when visiting the Merlin Fun website. Please follow the instructions to complete your booking for Madame Tussauds London.
On the day of your visit you will need to present your booking reference number and payment card to collect your tickets.
Alternatively you can take the voucher with you on the day of your visit and redeem it upon arrival to the admissions desk.