Dan

This is a question always asked and the answer I used to give was this.

If you're buying a 150 watts per quare meter system as it's hitting the rating required for a said room heat-wise meaning you can remove the rad.... then this will need to be fitted in all 90+% of the floor (everything other than the perimeter gap) so in those kitchens and bathrooms you'll find your heating is underpowered so the answer is buy a 200 watts per square meter system to counter-balance this effect.

If you're buying the heating just for a tile warmer.... then why stick it under the kitchen units? you'll not be standing there!