In the old days, UK casinos could get away with great-sounding bonuses that weren't good for the player.
The old bonus rules bundled your real money balance and bonus money together. The wagering requirements were usually impossible to meet. If you took a non-forfeitable bonus, playing and winning with real money became irrelevant.
Starting 31st October 2018, all UK casino bonuses have to be forfeitable bonuses. You can forfeit your casino bonus and withdraw your real money deposit and winnings any time you want.
Related: How to measure the value of a deposit bonus.