As Rilot said - luck of the draw. Can be days, can be weeks.
Customs is normally charged at UK VAT rate (so 20%), but what makes it unfair is that they VAT the shipping as well. Which means that if the CPU was £100 and the shipping was £15, they'll VAT it as if it's £115. Parcel Force also charges approx. £8.50 admin fee for holding your item. When it arrives at your local depot, you'll receive a letter in the post with a phone number to ring and a reference number to quote. You pay by card, then the CPU is dispatched and usually arrives on the next working day.