It could attract 6.7% duty and standard vat if it gets stopped by HMRC (and the extortionate and ridiculous Royal Mail £8 "duty handling admin charge").
Then again, it might sail through without a problem. Buying from the US and paying with Paypal makes the transaction itself pretty risk free. I have only been hit by HMRC on one out of about 10 incoming parcels in the last year.
I've bought odds and sods like lens caps and filters often enough with no problem.