If you want to work out the import duty and VAT you will have to pay on importing back into the UK then there are a number of duty calculators online that are free to use. You’ll need the HS code (IIRC 8518 for amplifiers, but most of the calculators will be able to help you ID the correct code you need) and also you’ll need to know the shipping and insurance costs. The duty itself will likely be very small, the big one is usually the VAT on the overall value.
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