My recommendation is this: If you're currently salaried, find out how to become an independent consultant to free yourself from full-time employment. If you do this wrong, it's worse than full-time employment. If you do it right, you'll make the same amount of money working half as many hours.
From there, identify pain points that still exist across multiple clients or leverage some insight you have into an industry you've worked in to create a business-to-business software-as-a-service product that generates recurring revenue.
It may take a couple years to figure it all out, but I have no doubt you can eventually launch a product that is bringing in the £4166.67 a month you want to save £1,000,000 over 20 years while also doing 20 hours a week in consulting if you still need that to cover your ongoing expenses. (I know a serious lifehacker who has that number down to 5 hours a week with a spouse and two kids, and he doesn't have a SaaS product yet. They're currently driving around the U.S. in an R.V. on a hunt for their perfect town to live in.) That would be on the low end of success in this space. It's also quite likely, you'll make much more in monthly recurring revenue and you can get out of consulting entirely if that's what you want to do.
There are a handful of people who have done this one way or another who are super open and transparent about it, which may be really helpful for you and other people who are interested in doing the same. A couple that come to mind are Patrick McKenzie and recently Josh Pigford. However, if you connect personally with other people who are doing this in chat rooms and such, you'll find that many people are quite open and willing to share numbers, tactics, advice, etc. if they feel it will help someone drum up confidence and help guide them along the way.
I think the first step is transitioning from full-time employment to consulting, because then you can scale your time commitment to people paying you up and down as you need to in order to build your business. And you never have to kick the coffee and wine habit you enjoy... just do another hour of work every two weeks.