Well I bought nearly $300 worth of stuff and I got charged just over £30 import duty +vat. I recon you will get charged roughly £20. Sometimes stuff gets through. Just the luck of the draw.
If Customs does check your parcel and you get charged tax you should add 26% to the total cost of your items including the shipping costs to roughly work out what it will cost you, this will cover all charges, tax's and costs.
general giude is approx 10% import duty + 15% VAT + "handling charge" levied by carriers.
the duty and VAT rate can vary greatly though as said, from as low as 0% for the VAT depending on the item. it all depends on what rate code the item falls under.
phone HMR&C, they are really quite useful and can be a quick way of getting the actual cost. did this when we were setting up our business 2 years ago and they were very helpful.
As bolter said, exhaust all UK possibilities. As the exchange rate isnt all that great at the moment. It can be just as cheap to buy from the uk, and in some cases now cheaper.
I only tend to get stuff that is very hard to get ie. no UK dealers for a certain make. And if its small stuff you can usually get it through customs no problems. Get the seller to mark it down so it comes under £18 and there will be no charges.