A separate import declaration is not necessary solely based on goods being provided free of charge.
Multiple items can be declared in the same declaration and box 42 allows for declaring prices for each item separately.
You can find detailed completion rules for SAD (C88) boxes on CHIEF here:
UK Trade Tariff: imports and community transport inwards
You can find guidance on how to declare customs value for imported goods here:
Notice 252: valuation of imported goods for customs purposes, VAT and trade statistics