Well, having just been through all of this with a frame from the states here’s what I know.
Your items are given a commodity code and that dictates the level of customs duty. You can find this on businesslink.gov.uk under ‘international trade’. There’s a number to call in Southend-on-Sea for clarification if required.
For a frame it’s 4.7% of the original purchase price including shipping charges. Once this has been calculated, VAT at 17.5% is added
You’ll only be liable for excise duty if it’s alcohol, tobacco or similar.
Once your package lands it goes through customs and is then picked up by Royal Mail/Parcelforce who pay the duty/tax on your behalf and then request the funds before delivery. This includes a standard handling fee of £8. Once cleared they arrange delivery. It all ran very smoothly apart from the frame being a piece of crap but that’s another story…
It actually worked out cheaper than I expected but you’ve got no control over the figure they come up with as it depends on their exchange rate calculation etc.
Cheers.