That’s how it works in the UK. All the basic tax info in your return is already filled out and you only have to give a bit of additional info which is very easy to do. The website is a standard website and is built with accessibility in mind so it actually works on Firefox for example.
This is true with the caveat that some special cases (like filing UK tax from overseas) isn't supported, or wasn't last time I attempted to use HMRC's site directly. You can use things like gosimpletax.com for that though.
