As an ex-brit that was once on British disability benefits, navigating the system can be crazy tricky. It is the kind of thing you need specialist help with and if you don't get it, you can get absolutely screwed. People don't think you need lawyer-like help to get around it but you absolutely do.
One trick they use is that they intentionally ignore the concept of good days and bad days. There are disabilities where you are more capable than other days and less capable on some days too. They ignore this and treat everything you put on the form as stuff you'd be able to do every day of the week. They don't tell you this though, in fact they word it as though they are looking for the most that you are able to do.