You should be able to use the exact one that is highlighted in your first screen shot. Scans may take longer with this one, but it should cover all theoretically possible transponders in the UK.
If you have a list of frequencies of all transponders in your area, you might be able to write your own list.
The full UK DVB-T list is stored in the file 4001.ini
You can find it in C:\Program Files\Plex\Plex Media Server\Resources\Tuner\Shared\scanners\dvbt\transponders
You could use that one as a template.
(You can also inspect this post Bug in Scanning DVB-T2 with QAM256 in UK - #2 by OttoKerner for a little more insight into the file structure. Though some of it may be outdated, because nowadays the polarization and symbol rate are no longer in there for DVB transponders.)
Save it also outside of the program folder, because it may get replaced or overwritten by a new Plex server update.
That must be completely down to your antenna and/or cabling.
Software cannot do anything about it.
