I can state that although we buy quite a lot of things from abroad, we have NEVER been charged when the value of the goods only has been less than £15 - even when (goods + P&P) has been over.
We HAVE been charged VAT when the value of goods only has been only a few pence over £15. (Actually, it worked out no more expensive in that case to abandon the goods to be returned to the shipper and re-order having made a tiny change which kept the price below £15.)