The value is whatever you put on it. A commercial invoice will need to be generated by the seller, this doesn’t necessarily have to reflect the purchase price, but it will be what the item is then insured for during transit.
In theory you could value it at a nominal amount, especially if it is a gift, but if it is lost then you won’t get a pay out.
Don’t forget though, the higher you value it, the more import duty will be applied.

In some instances, if it is your first import, you may be asked to complete an EORI application. Check this with the chamber first because it can hold up the import, and any hold will be subject to a daily charge.

Pete