As you also point out Whistle Blower it is quite common for FIs to work on a self employed basis. However I would say as a restricted FI needs to be supervised then in my opinion it is questionable if they could be truly self employed (the need to be supervised would certainly raise questions for IR35 compliance, this is only with reference to those intending to route payments through limited companies).
I would recommend to the original poster, do the CPL and FI first, get employed as an FI(R), get your restriction lifted. Once the restriction is lfted you could then start a limited company and begin working as a freelance instructor. Then you could easily argue the IR is a rating that is useful and required for your job. You could then put the expense of the IR against tax and claim back the VAT if registered.