In noticed this advisory that the National Gallery posted on X/Twitter earlier today.
https://www.nationalgallery.org.uk/visiting/plan-your-visit
Revised security measures at the National Gallery
Following recent incidents within the Gallery it is now necessary to introduce increased security measures to ensure the safety of all who visit, National Gallery staff and the nation’s collection of paintings. No liquids can be brought into the National Gallery, with the exception of baby formula, expressed milk and prescription medicines. We urge all visitors to bring minimal items with them including no large bags. All doors into the Gallery have walk-through metal detectors where we inspect bags and rucksacks. We anticipate it will take longer to access the Gallery and we apologise for this inconvenience in advance of your visit.