I signed up for 11plus.co.uk because it is well maintained. Forms, Payments and WordPress are up-to-date. The site is looked after and that means we are looked after too. You are doing a good job and I have seen competing sites that are toxically vulnerable.
Because you are handling personal information and money, can I request that you add good https support. It is easy, cheap and a matter of urgency. It will further set you apart from competitors and will be a great selling point.
This is something that can be achieved in days and for tens of pounds giving a great return for us and for you.