Not sure all of the facts are correct
"Denying me my commission payment of £100k (before tax) after unfairly dismissing me with one day's notice ... £25,000 was repayment of the overpaid commission"
Having worked in technical sales, I can tell you that commissions are never paid too quickly and are therefore rarely overpaid. But I can also tell you that it is not uncommon for people to be fired just before their commission is earned so the company can avoid paying it. Experienced salespeople always quit just after they receive their commission, whether monthly, quarterly, or whatever. And most salespeople are ruthless, with sales managers being ruthless squared.