Today many British gardens contain summer houses so the demand has certainly increased, mainly because of how cheap summer houses are in today’s market. They can often seem like sheds with windows, and furniture inside instead of junk.
However summer houses can actually be a lot different. The wood used is extremely thick and strong, while they are often well insulated, making them ideal for British summers and not just the summers in Spain or Greece. One benefit of summer houses is that you do not require planning permission, unless it covers half of the garden, which is not usually the case.
Summer houses can be great spaces for gym equipment or your own interests or hobbies like photography or art, and they can be built to last. If you feel as though your summer house would be regularly used and it almost work as an important part of your home then yes they are worth the cost, but if they are going to sit there, collect junk over time and act as a rather expensive and large shed, then they will certainly not be worth the cost.