If you have only 1 flight, think about a holiday gliding course of 3 to 6 days or so. Near S London would be Kent GC at Challock (I started there), Lasham or Booker. But as it would be a holiday and staying at the club pretty much any club in UK that offers holiday courses would be great. You build up a lot of flying and have fun the same time. Accomodation can be a bit spartan, but you are there to fly and have a beer or three. Ive done mini courses at Challock, Dunstable, Lasham and Deeside. Had a great time at all. If you choose somewhere with winch or autotow you get lots of take offs/landings which is what you need to practice.