It depends how the retailer is set up for this. What should happen is that they don't charge VAT on goods over £135. You will then pay VAT plus a handling fee on import. What you might find is that some retailers don't know how to handle this, and will charge you VAT and you'll also be charged UK VAT by HMRC.
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Looks like you'll need to pay VAT but no duty:
https://www.gov.uk/goods-sent-from-abroad/tax-and-duty
https://www.trade-tariff.service.gov.uk/commodities/8471300000?day=16&month=10&year=2024
Unless you're totally wedded to buying from Spain have you considered a UK seller?
https://starlabs.systems/ is amazing BTW.
I would love to buy from them and their new Starbook mk7 is actually my first choice but it's about 50% more expensive than Slimbook's equivalent. Starlabs would be much easier to get customer support from though and their model comes with Coreboot, which I would like. Perhaps I should by a scratch card or go down to Hampshire and try and haggle the price down!