Synairgen is a market maker stock and uses the SETSqx trading system. There is no order book. Any limit order would only execute if a matched limit order was uncrossed and executed at auction uncrossing times. These are 0800, 0900, 1100, 1400 and 1635. Even though you wanted to sell @ 175p and if the sell price was say 200p+ it would still not automatically execute if using limit orders - only if your trade was uncrossed (not crossed out) at auction uncrossing time would it execute.
https://www.lseg.com/sites/default/files/content/documents/LSEG_SETSQX_FACTSHEET_1.PDF