What do you mean by accessibility - do you mean their availability to workers, or their suitability for workers with disabilities?

Same as with any construction welfare "as close as is practicable" the regulations do not differentiate between rural or city building. If it turns in to a major trek to get to the welfare van from the work site it is not close enough and other arrangements should be implemented.

Hi gordon j - does this help? Provision of welfare facilities during construction work (hse.gov.uk)

Usually the question tends to be asked in relation to transient sites often in the middle of nowhere with a rule of thumb that you should be able to get to the toilet (and associated washing facilities) within 10 minutes whether on foot or with transport, but where other facilities (seating, eating, drinking and associted washing facilities) possibly being further away.

So, you might have satellite facilities (you mention a van) for the basics and have the main welfare facilities in a compound.

Same general principles apply to an urban location. In broad terms relying on the public toilet being open or going to the pub is not an acceptable alternative to the employer (or contractor) providing facilities.

However, if you imagine people doing minor works at domestic properties, the occupiers may be prepared to make some of their facilities available. 

P