LONDON, April 14 : A Pop-Up Hotel famed for catering for celebrities and rock stars at the Glastonbury Festival is making its British Grand Prix offering 'fully immersive' by adding a trackside swimming pool and palm trees to the experience. What might sound like an April Fool's joke, or bold gamble on English weather that in July can range from heatwave to a sudden opening of the heavens, promises to combine the thrills of motorsport with the pampered luxury of a holistic weekend retreat.