Look up the category (or closest equivalent) on the HMRC website for the item. It’s basically 20% VAT, plus the customs duty for the item (about 2.5% for effects pedals), plus a £12-25 clearance fee to whatever courier is handling the delivery, depending on the size of the item.
you might be able to put it under this code:
“Amplifier, consisting of active and passive elements mounted on a printed circuit, contained in a housing” 8543709030. 2% customs duty on this.