Will I have to pay customs charges and VAT when I receive my Thonk parcel?

Yes, because the UK has now left the EU customs union.
You should expect a similar level of charges as when you get a parcel from the USA.

You will be charged in the same way as Thonk customers in Canada, Australia, Japan etc. are. The postal carrier or courier will charge you for:

Duty – Typically 0-3% of the total depending on the specific products.
VAT – Charged at your local rate
Clearance Costs – Set by the postal carrier or courier. This is the fee they charge for collecting Duty and VAT for your government.