It's really tough sokel. Buying something ready done tends to bump up the price. I'd suggest a field shelter, different sizes are available, but for 20ft by 10ft you'd probably end up spending about £2k anyway. I've been looking at 12x 12ft field shelter, and for one that has what I want, skylight, kickboards etc, it still comes to £1052, which is still before delivery and putting it up!
Beth