The Offer Is Bigger Because the Rules Are Different
UK advertising rules constrain how a licensed operator can present a welcome offer, and UKGC affordability expectations constrain how much a player can chase one. Neither applies here, which is the honest reason the headline is 280% plus 235 free spins rather than the 100% up to £50 that a UK site of similar size would run.
The cost is in the conditions. 35x applies to the bonus, not the deposit: £20 in gives £56 of bonus funds and £1,960 of turnover before any of it converts. A maximum bet applies for as long as the requirement is open, and breaking it can void the bonus and everything won from it — this is the term that catches people, because raising your stake after a win feels natural and the condition is still live.
At a typical 96% RTP, £1,960 of turnover has an expected cost of about £78 against a £56 bonus. Claim it for a longer session, not because the arithmetic favours you. Declining is a legitimate choice and leaves nothing to clear.