Employers must “so far as is reasonably practicable” provide adequate toilet and washing facilities necessary for the well-being of your employees. This means you must provide such facilities unless it is clearly unreasonable in terms of time, trouble, cost and physical difficulty.
According to the Workplace (Health, Safety and Welfare) Regulations 1992, sanitary provisions should be:
Enough for those expected to use them
Suitable and readily accessible
Separated for men and women (where possible)
Well lit and ventilated
Obviously, you must always consider the needs of those with disabilities.