DIPPRIV4200 | Goods and services for the official use of entitled missions: purchases made in the UK
From HM Revenue & Customs · Diplomatic Privileges Manual
There are no provisions for relief from VAT and/or duty on taxable purchases made in the UK by entitled missions except for:
the purchase of motor vehicles (see DIPPRIV6000)
the withdrawal from a UK customs warehouse of alcohol and tobacco products (see DIPPRIV3800)
hydrocarbon oils (see DIPPRIV3600)
certain limited entitlements as granted by the FCO (see DIPPRIV9200)