Please don't do this without proper INDEPENDENT advice.
Two issues: -
1) These type of solutions are often sold as solving loose or slipping tiles on an older roofcovering. The trouble is that it locks them in place, making a single tile that does need replacing much more difficult as a job.
2) (and far more importantly) Condensation!! Doing this may make the roofcovering airtight - it depends on what ventilation you have at the eaves already (vent strips??). Water vapour rising through the house needs somewhere to escape to. It turns to water droplets in the roofspace and if it can't escape will drip somewhere back onto the ceiling joists. Selling this as an impervious insulation layer is (IMHO) a ridiculous idea - one needs some airflow through the roofspace OR a layer that allows water vapour through. Getting the airflow through is going to cancel out any useful value in the insulation being there.
Insulation is best put flat against the ceiling joists OR as part of a properly designed insulation system - the way an attic conversion has to be done to make the atticspace insulated.