Try to buy goods with a value less than £15.
But I know what you mean. I've had goods of low value delayed by Customs and also they have had simple errors on calculating the value. Once I ordered a supplement and the invoice listed the name, the strength and the value. They then ignored the value and charged me for vat on $1000 (1000mg being the strength)
From the website
http://www.hmrc.gov.uk/customs/post/buying.htm#3
All other goods
If you order or send purchased goods other than alcohol, tobacco, perfume and toilet water from a country outside the EU then you:
• don't have to pay Excise Duty
• may have to pay Customs Duty on goods with a value that exceeds £135
• will have to pay import VAT on goods with a value that exceeds £15
Note that on all goods from outside the EU, Customs Duty is waived if the amount of duty calculated is £9 or under.