If it comes through the mail, there's less likelihood of being charged, unless it's tracked and delivered by Parcel Force. As well as the VAT and Duty, you get a collection fee which was about £8 but is now £13.50, I believe.
If it's FedEx, UPS etc., you're likely to get the bill a couple of weeks after delivery, and the collection fee will be £20+. The VAT will be charged on the declared value and the shipping cost and, in my experience, this is grossly overestimated (so worth querying). Whether any Duty is charged, and how much, depends on the class of goods.