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Thanks for all the answers.
Some of you think I'm greedy. Sara mentioned the National Lottery, but this isn't the National Lottery, it's a less well-known website. If it were the national lottery, I'd trust them, they'd be open to public scrutiny. I'm not going to put the name of the website here because I don't want to accuse them of anything without knowing my facts. I didn't get a choice between the prize and the cash alternative, they told me I would get the cash (even though the prize is readily available to buy).
So just supposing that there's a website which always paid £200 less than the value of the prize. It's a profit-making website: they are paid by their clients to collect data from prize draw entrants, they're not giving away prizes because they're really generous. So if they're paying out less than they should, that's fraud, they are defrauding their paying clients and they are defrauding those who enter the prize draws. People who enter online prize draws invest lots of time in entering lots of draws, in order to increase the odds from minuscule to meaningful. For all we know, the person at the other end could be transferring £600 to the winner and £200 into their own bank account.
So call me greedy if you like. The terms and conditions are vague, they don't say anything about the amount of the cash alternative or how that amount is arrived at. But if you award someone a prize, tell them they'll get money instead, but the money isn't enough to buy the prize, that doesn't seem right to me.