According to HMRC charges are levied in UK as follows:
Tax and customs for goods sent from abroad https://www.gov.uk/goods-sent-from-abroad/tax-and-duty
£39 is the "gift" threshold..No charges by HMRC. Anything above that I would not purchase from outside UK for obvious reasons. The risk can be costly and including messing around with Sellers/RM/Parcel Force et al.
I use the term Scam because as regards the internet too much of it is precisely that.
After Note:
AlieExpress is applying VAT to all purchases as far as I see and looking at other sellers on that site. Which means the £39 gift thing I mentioned above no longer applies. It probably does on holiday etc travels through UK ports.
What I don't understand is how the VAT actually gets to HMRC having paid the China seller via the various payments methods.