I'm thinking more along the lines of being quick to add one tomorrow surely they allow you at least 5 minutes to checkout. I wouldn't expect to have one reserved right now.
Nope its a straight race to get payment authorised. OCUK allocate stock as payment gets authorised, not according to order time or number. Stock is not reserved when its in your basket, you have to be as fast as possible and pay with something that authorises instantly.
The quickest method is to buy ocuk gift vouchers now and have the balance added to your account and ready to go by 1:45pm and start refreshing the page of the cpu you want like a madman. Course if they bump the price up again between now and then you're stuffed so you better get £50 more than you think you need lol.
Second fastest method would be either credit card, paypal credit or paypal balance.
Third fastest is debit cards which have to check your available card balance before authorising
Next its PayPal card processing which has to connect to visa/mastercard, pass through your details and then check your balance.
Finance and bitcoin are the slowest as they need an actual person at the store to bother checking they are authorised before approving payment (or did last time I used either which was some time ago now admittedly).
[EDIT - No refunds on gift vouchers, while they are the quickest way to checkout once bought you've committed that money to your account here if you aren't quick enough for whatever reason...such as the site lagging out for example.]