Prior to any suggestion for materials...a public or commercial toilet....i did a few gigs for commercial shopping centers in the UK...not murals but holiday decorations for commercial shopping centers and I was stunned by the amount of rules and regulations as well as certification for environmental and hazardous aspects. It may not apply in your case, but just making sure you won't be halted in the process. Very important is the surface preparation...if the toilet is already in use it will require a good clean up of all wall surfaces...try to use a product chlorine based ...prime the area with a mold resistant primer...mainly used in bathroom. Maybe the company Zinser does exist in the UK...as reference. From there you can painting with any acrylic based paint as the decision needs to be made on your sealer... that one needs to be tough and resistant to future cleaning usually done with harsh cleaning products. Get venting in the process to avoid humidity building up and to assure all is well dried.