@haggis_mince thanks for a really helpful explanation.
If fractional shares are essentially derivatives why do we have to pay stamp duty? The legislation for stamp duty is more precise than the more vague legislation for share isas (which simply refers to shares) and I don’t believe (I could be wrong) that stamp duty is due on derivatives. Thus if HRMC want to argue that fractional shares aren’t allowed in an ISA then doesn’t it follow that stamp duty isn’t due (if buying fractional shares in Invest accounts)?