Covering a tiled fireplace without damaging tiles (UK supplier needed if poss)?

You can actually purchase Tile Paint. It's a type of paint that is specifically made to cover tiles. It covers more evenly and better than regular wall paint and you can always strip the paint if you decide you want to go back to the original tile. You can also get decals to stick to certain tiles to make a design. There are also ways to texturize the paint to give a different look.