Robroy , the Portaloo or Tardis/Turdis has been there for years , I think they put it there for when the main building was shut at night , in pre-Covid days .
Ironically I saw every lay by from the Warminster bypass until Westbury full of lorries last night at 19.00 after a day on Weymouth beach .
You may have been better off at the new garage on the Frome bypass where the Travelodge and pub is , an industrial estate , the garage is open 24 hours and has a loo that is open , off Jenson Button Avenue .
Or Beckington services has a garage with a loo, 24 hours , there is a lay by going towards Trowbridge , depending in which way you were heading , no place to turn around if facing towards Trowbridge where you can U turn at Southwick , 6 miles away from that lay by .
Tea van and hot food in that lay by off the A36 .