James,
Your comments about royalties is very interesting. I'm a Yank so I know nothing. All I know is that BOI wants you to show "passive" income not "earned" income {salary, wages}. I do not know how the UK classifies royalties. In the end it isn't even important what you or even the UK Government says, it's how the BOI interprets it. You can be absolutely correct but if the BOI doesn't agree then you're sol. Also I'm not sure why they ask for tax returns. To my knowledge none of the requirements say you have to show taxes paid. There's a lot of very wealthy people in the US who don't pay any taxes.
Based on your age I assume you have qualifying health insurance. If not you should probably be able to get it. That's a stumbling block for many applicant (ie older ones 70+).