I think S17 will be seen as a peak performance year for a number of UK airports. The continued uncertainty of Brexit (eg will Brits have visa free travel to the EU come March 2019?) will have an effect on people's planning as will a slightly gloomier economic outlook and the weakness of the Pound. I think people will still take their Summer holiday whatever the cost but will cut down on short breaks / weekends away etc.
On the point of 10% seat capacity reduction S17/S18 (as it stands now) that is indicative of the over-supply from S17.
I know many aviation enthusiasts will say 'yes but the planes were full' but clearly they were full of too many people not paying an economically sustainable price - hence no more Monarch and all the other airlines exiting. Take it to the ultimate extreme and give seats away for nothing you will have almost 100% load factor.
I would not expect the 10% lost capacity to be fully made up as that would imply another round of over-supply, losses and more tears next Winter. Some more capacity will inevitably appear but the full 10% would be daft and ultimately undesirable if we want sustainability over the long term.