You can sell your shares assuming the underlying stock market is open, and a quote is available for your share from the market (for example, the shares are not suspended) at the time your order is "actioned", i.e. submitted to the stockmarket. Currently, all orders are actioned at or soon after 3pm London time each business day. 

FYI: typical stock market opening hours are 08:00 to 16:30 London time for UK/EU shares, and 09:30 to 16:00 EST for US shares, although this may vary, for example during market holidays.

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