There's no import tax on mobile phones but you'd be charged 20% VAT on both the purchase price and the carriage (totalling £81.60) plus a £8 inspection fee, making £89.60 altogether.
However there shouldn't be any shipping charge because shipping is free for orders over £60:
https://oneplus.net/uk/oneplus-3t
Even though the website states that UK consumer protection legislation applies, I'd still be wary of buying from Hong Kong. Further, I'd think twice before buying any mobile phone which has non-replaceable battery (as that one does).