The acquirer of the equity instrument is subject to UK stamp duty. The UK stamp duty is automatically withheld and DEGIRO passes this tax charge on to its clients.
In general, UK stamp duty applies to shares issued by a company established in the UK or with a share register in the UK. Eligible securities listed on the London Stock Exchange's Alternative Investment Market (AIM) segment are excluded from Stamp Duty.
The taxable event is the purchase transaction.
The tax rate is 0.50% (as of 2020) and is calculated on the transaction value.