Here's HMRC's guide for royal mail/parcel force if you haven't already seen it:
http://customs.hmrc.gov.uk/channelsPort ... P142_10261
The tax free limit changed from £18 to £15 in November.
I'm still not entirely sure if shipping is included though as in 2.3 it states:
an intrinsic value not exceeding £15, will not be charged any duty or VAT
and then in 3.1:
goods over £15 in value, and on gifts that are over £40 in value. The value of the goods for import VAT is based on the:
■basic value of goods, plus
■postage, packing and insurance
Judging from that I'd say £15 including postage, though I'm not sure about their use of the word 'intrinsic'.
I've heard a lot of people not getting caught by customs, but if you do, especially for low value goods, the handling fees will probably cost substantially more than the tax itself!
Sorry that probably wasn't much help, but I would also be interested in hearing about other people's experiences!