There's no import duty on items where the value (not including transport costs and insurance, etc.) is under £135, on camera gear duty is usually around 6.7% of the item's value. VAT of 20% applies to items valued over £15 and what's even more fun is it's calculated on the value of the items, the shipping costs, insurance
and any import duty you've already been charged.
Long story short, if it's under £15, no further charges apply otherwise time to get out the calculator. I remember ages ago when I put in a big order with 7dayshop I received a number of small packets, the value of each being under the VAT threshold. I suspect that loophole's been closed though.
Source.