I would guess as it covers OCUK for people changing their minds.Seeing as you've proven you can make the deposit price less than the cost the console will most likely be, what is the reasoning for keeping it so high?
If OCUK buy in 500 Xbox One for launch day based on pre orders from 400 people at £50 and then the day before 200 of those decide they can afford to lose the £50 because they found it £100 cheaper, or with a better bundle or whatever it potentially leaves OCUK with a warehouse full of launch price Xbox One they can't sell. At £300 deposit if you change your mind and lose the deposit their costs are covered and they can order with a degree of confidence that you're serious.
Of course if they have a sale or return agreement with the disties that's all rubbish and they'd just like the money in the bank, which as it's your choice to order or not is fair comment I suppose.
Just guess work on my part but that's the way it's worked in the past with retailers that take a large deposit.