Plasterboard does not need priming before tiling if you are using a ready-mixed tile adhesive such as Evo-Stik Tile Adhesive.
When tiling onto bare plasterboard there is no need to prime the surface if you are using a ready-mixed tile adhesive such as Evo-Stik tile adhesives. You will need to use scrim tape to cover all joins in the plasterboard to prevent adhesive and moisture passing through the gaps.
However, if you are tiling a shower area that will be exposed to a large amount of moisture then British standards require the area to be tanked with a waterproof membrane to prevent moisture passing into the plasterboard.