For the past few weeks the atmospheric conditions have been such that there's been a lot of interference to radio and TV signals from out-of-area transmitters, both in the UK and abroad. (For example, reception of Classic FM in my car has disappeared several times beneath a Dutch or Belgian station on the same frequency). There may well have also been some solar flares or other occurrences which can similarly affect reception.
All TV aerials are better at receiving some frequencies than others. Yours clearly isn't great when it comes to handling the frequency used by your local PSB1 multiplex (= 'bundle of channels'), which brings BBC1 to you, at times when the signal strength is poor (because, for example, of interference from other transmitters).
A couple of cheap signal boosters
https://www.argos.co.uk/sd/tv-signal-booster/
plus a couple of fly leads to connect between them and your two TV sets
https://www.screwfix.com/p/fly-lead-coaxial-plug-to-plug-1m/41720
should fix the problem. (Ideally you should use just single two-way signal booster, which would be cheaper, positioned before the down lead from your aerial splits to serve the two separate TV sets - but that might mean crawling around in your loft and finding a way of getting mains power up there to power the booster. So I've gone for the simpler option!)