I would expect the old tiles to be taken up. Also it is normal to move toilet and tile under it not around it. The biggest thing that will effect the total cost is the new tiles you choose - measure the bathroom floor area and then when you choose the tiles you will know how many to buy and how much it they will cost. Plus something for the grout. Budget £300 for the tiler and hopefully you will get change - do get a fixed quote before the tiler starts the work.