Great idea, but: Why do I need to login to answer the questions? Honestly I would want to decide after filling in my budgetary details and getting a result whether I want to share it with you or rather not. Privacy is a big issue when it comes to personal finances, at least for me.
Thanks for your input, the website is now usable without signing up.
I would remove the login requirements to generate a "budget" until you want to save that budget. You could also have additional features that are unlocked only after an account is created. Also, I would offer other currencies in addition to the £.
Thank you for your input, the website has been updated so that you don't need to login to try it. (and a temporary user is created so as long as you keep your cookies, it will be saved). If you decide to create a user afterward, your data will be transfered to the real user.