I emailed them to ask why couldn't they have a non-subscription based model, this is their reply:
"Our system is indeed a bit different than the Boris Bike system: for example we don't have docking stations, you can collect your key and bike from the train station. This results in a lot of freedom: there is a fixed fee for 24 hours, and you don't have to search for docking stations every 20 minutes to prevent your fee mounting up. So the setup of the schemes are different, and this also results in different type of subscription and payment.
From our customer research we noticed that the price is not a big barrier for people to use our scheme. The £10 for an annual membership that allows you to hire bikes at 70 locations is actually perceived as quite low. But we are constantly evaluating our product so in the future we might want to change something to our pricing system."