VATFIN4400 | Securities and related services: member firms trading on the London Stock Exchange (LSE) - principals or agents?
From HM Revenue & Customs · VAT Finance Manual
Share transactions conducted by member firms on the LSE will be exempt under item 6 of Group 5, Schedule 9 of the VAT Act 1994 if the member is acting as a principal or under item 5 if the member is acting as an agent, (provided the intermediary criteria are met - see paragraph 9.1 of VAT Notice 701/49 Finance and VATFIN7000). The contract note appertaining to the share transaction must state which it is.