Customs Duty varies greatly between different commodities. For some items there's no duty at all, or it's very low, at, say 2½%; with others duty can be far higher, at perhaps 10% or 15%. So we can't provide you with an accurate figure unless you tell us what it is that you're considering purchasing.
However, as most items attract VAT (even if there's no Customs Duty on them), you can expect to pay at least £138.40 in VAT and a further £8 for the customs examination fee.