If £15.00 GBP or under, there's no charge. Above that, which I'm guessing you've comfortably exceeded:
20% VAT, applies to both the value of the goods, *and* any shipping charges.
If the total value now (ie. goods plus shipping plus VAT) is more than £135.00 GBP, it *may* be subject to import duty too. Duty rates are pretty variable, and depend on exactly what the contents are. I would strongly recommend you ask the seller to complete the customs sticker with the description:
"Scale Model Assembly Kits"
"Commodity Code 9503003000"
That specific description is legitimate for the goods, and the precise category is 0% duty. (Toys are generally 0% or 4.7% depending on the specific category, so making the customs form absolutely clear is handy – if it just says "models" without being sufficiently accurate, it can be pot luck what it gets classified as….)
Similarly ask them to be clear on the form what the declared value covers (ie. put "$xxx plus shipping", or "$xxx including shipping", not just "$xxx"….) Inspectors tend to assume the declared value is *just* the cost of the goods, and add the shipping on to it, so it's important to be clear, or you can end up being charged double VAT on the shipping costs.
Finally there'll be a handling charge to the shippers for the privilege of relieving you of your VAT. Royal Mail that's £8.00 GBP, Parcelforce depends what service it arrives by, but usually also £8.00 GBP, sometimes £13.50 GBP iirc.
Dom.