One possible direction would be something like Binance card, which you may top up with fiat whenever you want or top up with crypto (depending on what you expect on short time - increase or fall of the price).
But you should have to study more this case, since you may have to also pay tax for the BTC you're selling. I don't want to scare you off, but you should see how is your case seen by the tax authorities in your country (UK?).