For goods moving within the EU, import duties and taxes do not usually apply. However, for goods imported from non-EU countries, Customs charges do apply, these may include import duty, excise duty and import VAT. Goods with a value exceeding £18 (for commercial items including internet/mail order purchases), or £36 in the case of gifts between private individuals, are generally subject to Customs charges. Normally charges are calculated upon the declared value (plus shipping costs for commercial items).
I'm pretty sure the sender only needs to sign the declaration sticker if the item i