>>> or are these customs charges?
They certainly are for anything coming into the UK from the EU (or from elsewhere, such as the USA).
For anything up to the value of £135 (excluding alcohol, tobacco and perfume) there's no Customs Duty or Import VAT to pay. Under those circumstances, there's no customs inspection fee to pay either.
However from £135.01 upwards, the first thing that happens is that Import Duty is calculated. (For some types of goods, that might be zero. For others though, such as cotton-based clothing from the USA, that can be as high as 27%). Then 20% VAT is added onto both the cost of the goods and the Import Duty. (Yes, that really is a tax on a tax). Then a £12 customs examination fee is added on top of all that lot.
The foregoing assumes that the items weren't sent as a gift. There used to be a 'gift allowance' that entitled someone in the UK to receive occasional gifts from overseas (such as those for birthdays) without paying VAT or Import Duty. i.e. the whole point of it was to create a concession. However the gift allowance now has exactly the opposite effect from the one originally intended, as any gift over £39 is subject to VAT (and consequently to the £12 inspection fee on top of that). So gifts valued between £35.01 and £135 get clobbered for charges, whereas other imports within that range of values don't!