Assuming the SLR is digital it should attract no Import Duty (unless it is classed as a video camera, in which case Import Duty is 12.5%. This classification is dependent on the framerate and resolution of the capture). A 35mm film SLR attracts a rate of 4.2%.
VAT is of course charged at 15% and a Customs processing fee of around £20.
I wouldn't think of buying anything from the US. It's a con and you know it. They ship used defective products to us knowing we'll not pay the return postage. The last item I purchased had 14 rubber stamps on it, 14 companies all taking a cut. Even the Post Office demanded a fee to release it. The product I purchased didn't work. Bitter experience but you live and learn.
onions: that really depends where you're buying from. I've ordered international items in the past with great success.
However, with the additional cost of VAT and processing fees, I think I will wait for another camera that is sold in the UK to come down in price a little, as the prices will match.