What stage is the build at, are the walls up?
generally the concrete for a raft foundations wouldnt be suitable anyway to tile on anyway.. the aggregate would be to big. Also the extent of the pour would be so large as to make it extremly difficult to get a level screed off.
The usual procedure would be to pour a secondary scree over the raft once th eheating pipes etc are installed. This secondary screed may only be about 60mm, or even as low as 30mm with an easi-screed type product.
have you the room to pour a screed?