british standards state 3mm for floors and 2mm for walls you could use a 1mm spacer if you want aslong as the tiles have rectified edges (ie edges are totally flat and all tiles are exact same sizes
overlay current floor with 12mm exterior ply and screw down every 300mm with 25mm screws (turbo gold screws from screwfix best) as that length screw will not puncture any pipes under floor boards and make sure floor boards are sucure before plying
prime ply with an acrilic primer (bal apd)...
take off the old tiles and have a look at the plaster behind. if the tiles had been fixed properly before theres a good chance plaster will come off aswell in places. check rest of wall with a coin and if it sounds hollow that will need to come off aswell because if you tile over it the weight...