"I'm a 35 year-old software consultant. I want to be a banker"

There is a process for getting into a bulge bracket M&A, go to the right school, do the right extra-curricular, get the right grades and get the right internship. You may have all the attributes for being successful in M&A but you are unrealistic to believe that you can redefine the defined process to fit your desires (note there are exceptions, are you well connected, have you sold your business using a target bank as the intermediary etc). That aside I know plenty of folks who didnt make it into M&A and are doing amazing likewise I know plenty of folks who exited M&A and are doing great.
Bottomline you missed the boat but you can do just fine outside of M&A.

No bank on the street wants spend money/time to train up a 35 year old software guy who comes with all his own ego and preconceptions. Your experience in Consulting basically counts for nothing and in most likelihood you won’t be able to stand the heat of the job and will leave after a month if you make it through the first 110-hour work week.... Selling software to CTOs that don’t know any better in an uncompetitive industry is completely different to my job pitching a merger to a fortune500 chairman with multiple banks trying to elbow their way into a senior role whilst an activist tries to unseat half his board. Did you go to Cambridge? Do you wear Grensons? Did you even think of either of those questions? If not you are not geared for this. Worst still, textbooks of Corporate Finance are as worthless as the paper they are printed on.
Banks want people they can mould into deal machines and rainmakers.

If I were you, I’d stay in IT with the safe IT jobs surrounded by mediocre IT people. IT consultants still get paid ok (compared with the UK average, whatever that measily number is - LOL!)

Hey - I got hired into banking at 37. Had tried to get into the industry for years and finally needed to go to business school to do it. Was recruited by a bulge bracket in the standard MBA recruiting cycle at a top ~15 US program. If you’re trying to lateral in as a VP or Director, you’re going to have a very very hard time. If you’re going the MBA to Associate route, I don’t see any challenges that your peers wouldn’t face. On the job, I haven’t encountered any ageism (that I’m aware of).

Ageism is the most widespread form of discrimination in banking yet no one cares. Maybe you should start your own #OldLivesMatter movement.