The minimum value of a buy/sell order is 1 EUR/GBP for our Invest/ISA accounts.
You'll not be able to execute a sell order where what is left of a financial instrument is worth less than 1 EUR/GBP.
You're holding a share with ABC, worth GBP 10. Then you decide to make a sell order for GBP 9.9 worth of ABC share. The platform would not allow such a sell order, as it would leave you with GBP 0.1 worth of ABC share, which would be less than the minimum of 1 EUR/GBP. In this case, you'll have to sell the complete share or an amount equal to or less than GBP 9.
*In exceptional circumstances, you might end up with a financial instrument worth less than 1 EUR/GBP, where a sell order below the amount would be allowed. This might happen as a result of corporate actions or past leftovers.