Write and tell them you will be filing a claim in small claims court in 30 days unless you receive a satisfactory response. Look and see what the cost of an economy ticket bought in advance would have cost compared to your business class ticket and claim the difference. It is very easy to claim... You can do it online and the costs are minimal. You get your costs back when you win, which you will.
https://www.moneyclaim.gov.uk/web/mcol/welcome