There could be an issue with your aerial or cable which I have no idea how you could check yourself but if no one chips in to help you could use Restore TV, an independent free programme created to ensure that all UK viewers continue to receive free to view TV or are offered a suitable alternative, if new mobile services cause TV interference, which solved my friends problem after she paid to have her set up checked and was told it was all OK.
I must admit I was annoyed that she had paid out good money to someone who did the least he could get away with and took advantage of an elderly and vunerable person. However assuming that the cable/aerial were OK I used the following site and we found that a recently installed phone mast was to blame and the filter they sent solved her problem and she got back the missing channels and the blocky interference on other channels.
https://restoretv.uk/