It's an aerial problem!!!
TV channels are grouped into 'mutliplexes', with each multiplex using a different frequency. In an ideal world, the elements of an aerial should be exactly the correct length to 'resonate' with the frequency that aerial is designed to receive. So, theoretically, you need a separate aerial on your roof for each group of channels.
That's clearly impractical, so we use aerials which are only a 'rough match' to each of the incoming frequencies. That means than any given aerial will be better at receiving some groups of channels than others. When the signal strength is good that doesn't matter but if the signal strength drops you'll lose the channels which use a multiplex the aerial isn't very good at picking up.
Start by checking here
http://www.digitaluk.co.uk/operations/planned_engineering_works
or here
https://ukfree.tv/maps/live
to see if your local TV transmitter is temporarily operating on reduced power.
If it's not, the chances are that either:
(a) there's a loose connection somewhere on your aerial cable (such as where it plugs into the TV) ; or
(b) it's been knocked out of alignment by the recent storms.
If it's (a) then all you need to do is plug the cable in properly.
If it's (b) you might need an aerial fitter to realign your aerial but, given that you still receive most channels OK, a simple aerial booster (around a tenner in larger supermarkets) should fix the problem.