IG terms and conditions say the following :
(2) We guarantee that when your agreed stop level is triggered in accordance
with Term 13(3), we will, subject to Term 4(11), close a Limited Risk Transaction at
that agreed stop level.
(3) Your agreed stop level will be triggered when our bid price (in the case of an
order to Sell) or our offer price (in the case of an order to Buy) moves against you
to a point that is at or beyond the agreed stop level.
So I guess the guaranteed stop loss is also valid when you send a “sell at market” order before the trigger, but the market price really achievable at this time would be worse than the guaranteed stop level. However, it would be interesting to have this confirmed by IG.