UK National Tide Gauge Network data was de-trended to a base year of 2017. This ensures that all historic events are comparable in magnitude to present day events. Extreme sea levels provided are also to base year 2017 therefore are for probabilities of events that could happen now.
Sea level rise should be taken into account when predicting flood risk in future years beyond 2017. Information on this is provided in the supporting documentation: Coastal flood boundary conditions for the UK: update 2018, User report.