> I am guessing that I could start at an entry level role and work my way up to 100k p.a. or close in 3-5 years

I wouldn't say that's super likely. Given that you haven't worked as a professional software engineer for ~10 years. There's more to being a senior engineer than being able to do challenging programming problems. Not to discourage you, but getting to £100k as a SWE in the UK is tricky, outside of finance. Even more so with 3-5 years of experience. That being said, it sounds like finance would be a good fit for you?