Duty (I think) is in the region of 5 - 6.5%, but VAT will be charged also. The formula is cumulative too:
Price of part + shipping = value of part
Duty charged at Value of part + %duty
VAT charged on this new value of part + shipping + duty at 17.5%
Import couriers will charge between £10 and £35 for doing the paperwork to allow the government to rob you of your hard-earned.
When you look at buying parts abroad, you will always run the risk of this extra cost being applied, so figure it into your calculations anyway. Postal services usually can't be bothered with small amounts, but they will happily collect from larger value packages and usually want cash at the door!
Courier services may not charge you on delivery but will send you an invoice for the additional charges later.
If shipping from USA, always ask me - I ship large, have my own freight forwarder who is working for me and you don't get involved in anything apart from paying me a fair price for the effort involved - no nasty surprises, big carriage charges you weren't told about etc.
Any method of avoiding duty and VAT on import goods is illegal - written down invoicing, "sent as gift or sample" or simple smuggling. I don't condone any of them but sometimes it's hard to say no!