Part G1 of the Building Regs state that adequate washbasins will be provided in any room where there is a toilet; or in a room or space adjacent to the toilet.
But any such room or space must be separated from any area used for food preparation. G1(2)
It applies to new installations in old properties though - if you want to put a downstairs loo where there wasn't one previously, you will need to adhere to the building regs.