(a) If it's a watch then you should have no "duty" (specifically Customs Duty) to pay. It is (or should be) all VAT, not "duty".
(b) The VAT payable will be 20% of the value of the item, including postage, plus a handling fee of £8 to £15 or so (depending on the courier).
(c) Whether you have to pay before or after delivery depends on the courier. Royal Mail and Parcelforce will require prepayment whereas other couriers often (but not always) invoice you after delivery. If prepayment is required then you will receive a letter in the post requesting payment, and you can then pay online.