Usually you only pay the customs charges once it arrives in the UK and it should be approximately the same as VAT (ie about 20% of the value).
I usually choose UPS. What you need to do is watch the tracking info and wait for it to land in the UK. You can only pay the duty once it's in the UK.
If your phone number is included in the delivery instructions, UPS will usually phone you a couple of days before it arrives (just to ask if the item is for business or personal use -- They can't take customs payments at that point).
Confusingly, the tracking info will include the destination airport as a location before it arrives in the UK. This is just a UPS customs activity log and not the actual location. For example, I have an L3+ arriving tomorrow and the tracking info shows this:
Soon after this you should see it moving again (usually via Thailand, India and Germany I think). When it lands at the UK airport, phone UPS with your tracking number on 03457 877 877 to pay the duty. Or just phone them at 8.00am on the morning it's due out for delivery.
If you fail to pay the duty on time, all that will happen is either, they will contact you to let you know the parcel is being held until the duty is paid (sometimes they'll still deliver the cheaper power supply and send you a separate invoice for the duty on that) .... OR, the delivery driver will ask for payment when he turns up at your door, which means you then have to make him wait while you phone UPS with the payment details.
EDIT: You will usually have 2 tracking numbers (one for the PSU and one for the Antminer). Make sure you give both of those to UPS when you phone.
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