Tiled floors are a major heat loss.QUOTE]
That is not correct. The insulating properties of carpet or timber flooring are negligible when compared to the insulation which would be beneath the floor.
In any case, the house here has underfloor heating so the floor covering should not perform any insulating function. In fact the tiles will act as a heat sink and release the heat more evenly throughout the night.
Also depending on the type of roof structure, it may not be advisable to go blocking up soffit vents as these may well be requires for ventilating the roof.
I'd agree with Superman on the thermal imaging. It'll point out the areas of greatest heat loss and confirm or dispel your suspicions. Google local providers in your area.