BAL Flex-One is my go to adhesive for these kind of jobs…
Can bed up to 15mm.. open time of 45 minutes (more like an hour) and working time of 2 1/2 hours and can still be grouted up after 6 hours
There’s not reason you can’t use that Mapei product but no self respecting tiler would buy his or her gear from B&Q
Get to a CTD, you might pay a bit more but it’s the right stuff for the job..
Or if you are looking for cheapest option then Go wherever is cheapest.. but don’t be surprised if it doesn’t perform