Customs valuation for EU imports is governed by EU legislation and guidance; the latter is available via the ‘Europa’ webpages.
Both the UK and the EU follow World Trade Organisation (WTO) rules on customs valuation.
UK rules can be found via this link:
Notice 252: valuation of imported goods for customs purposes, VAT and trade statistics
They are similar if not identical to EU rules.
This includes ‘free of charge goods’ in section 10.