If the UK customs handle it anything like the one here in Austria, then the shipping does not count towards the import VAT limit, i.e. you can order goods worth 15 GBP plus shipping on top without paying import VAT.
Reading Springfairy's post ("may have to pay Customs Duty on goods
with a value that exceeds £135"), shipping should also not count towards the customs duty limit, i.e. you can order goods worth 135 GBP plus shipping without paying customs duty, only import VAT.
BUT, once the customs officer decides, that you have to pay customs duty, the whole amount (goods plus
shipping) will be used as the calculation basis.
This is how I manage my SN addiction for the past 3 years now: I limit my orders to the USD equivalent of 22 EUR - depending on the current exchange rate this ranges from 29 to 32 USD (excl. shipping) - and that way I never had to pay import VAT or customs duty. Shipping is calculated based on the weight of the goods and there is no longer free international shipping on orders over 125 USD since January. I figured that I rather sponsor the US postal service than the Austrian fiscal authorities
and as a bonus I gathered a huge collection of the weekly GWPs, which - as we all know - are priceless
Hope that helps!