Okay thanks.
My problem, on your behalf, is thinking about how you get to the point of earning a reasonable living quickest knowing how you are just one of many looking to achieve the same.
My thought process is, as a competent engineer, could you not simply set yourself up as a 'wet plumber' in the short term in order to keep your family and then over time do your gas - if in deed that was where you decided you wanted to go?
Why? Well frankly, by doing gas all you will be able to do initially, without any form of reputation, is compete on price for jobs. Never a good basis for a business.
Wet plumbers however are just as skilled (I'd actually say more so than you'll be as a new gas engineer) but with a much much broader remit. The other issue, for me anyway, is that as a wet plumber you can create a USP for yourself as presentable, caring, punctual etc which you are not really going to be able to do as a new gas engineer - IMHO.
Personally, I've never had a desire to do gas. I saw it as far too competitive and I was unable to create my niche of working for mainly older people.
Of course this view is simply that, a view. However don't dismiss other opportunities out of hand. You are used to assessing situations so look at this alternative view in the cold hard light of needing to keep your family.
Happy to chat further offline if it would help.