Toilets
Many churches already have toilets somewhere in the church complex. An easy solution is to fix it with a combination lock with a small sign 'Please text to ..... for combination code'.
Some churches may use the Church Walking Pilgrimages movement as a reason to build a toilet within the church, using grant or crowd source funding. The Diocese will advise on the best design and location, taking into account archaeology, burials, water connexions and other issues..
Two ways are possible if a flush toilet is not possible; Trench Arch toilets, where if drainage is good and use low, the waste percolates away or compost toilets. You can build one yourself, see for example the Centre for Alternative Technology Compost Toilet design sheet or you can buy one 'off the peg' e.g. Dunster House Composting Toilet or Natsol: https://natsol.co.uk/ or search for others. Often you buy the toilet and building separately, so are able to choose/make the building to suit. With compost toilets a below ground pit of 0.8m depth about 2m by 2m is necessary, so archaeology will have to be considered. Planning Officers generally prefer the toilet not to be attached to the church, but to be a small separate building. Toilets in churches require Diocesan Consent (a 'Faculty), and toilets outside churches within the grounds require a Faculty and Planning Permission. We have a lot of information at this site on toilets, so please get in touch.
Trench Arch: see Diocese of Gloucester: waste water from churches
Trench Arch: https://missionandmortar.hereford.anglican.org/categories/local-mission/ download .pdf Kitchens and Toilets
Compost, see Natsol for plans and methodology