You can’t. Seriously. No strat house values practical experience over first class academics. If you don’t have the A’s then no amount of practical application will make up for it.
Also "self-employed" is kryptonite to a blue-chip strat house. Their entire internal business model is based on hiring people who are so "invested" in the job that they will work 20 hours a day thenfly to Hong Kong for a 10 minute briefing. People who have been self employed do not get themselves trapped into that model.
If you really want to work in strategy (but not consultancy) then your best bet is to migrate into an strategy role in industry – preferably your current industry. This will not let you move to a strat house, however. That door is closed and bolted.
You could go do an INSEAD MBA, spend 50K doing so, come top of you class, and they’d probably still ask you what A-levels you got.