If there is enough slack on the coax and your able to cut an inch or so off and the dialectic (the insulator around the copper core) and its turned from white to yellowish, dampness has got inside the coax. Although this can happen at any point.
Air dialectic is RG6 is the cheapest 75 ohm coax. I’d recommend you use low loss RG11 if your going to renew the coax from the aerial to booster. RG11 is a higher grade providing more air space in the insulator and therefore can higher frequency giving better signal quality than RG6 and less likely to suffer from more signal loss, giving a poorer signal strengh. As wigwam says use amalgamating tape around the external F connector or put grease or use dielectric grease with a F boot.
The likely hood your booster has gone faulty is realitively slim – you could rig up a coax extension connecting to the incoming coax, directly to the TV, thus illiminating the booster.
You can check for dead shorts using your multimeter. There are special meters that SWR/impedance analyzers that will downstream of the cable from the F connector, albeit too expensive for a small one off job. So don’t go pulling your hair out if after re aligning the aerial its not much better. It may be wiser to install new coax and then realign the aerial.
By your mention of North Wales I was thinking Rhyl or Prestatyn aera lol. Barmouth yes I’d say Presli is the correct transmitter, however you can enlarge the map in the link below and click on each transmitter for coverage details. However its possible to align to full HD transmitter with the right bits but, guess thats OTT for a caravan.
https://ukfree.tv/maps/freeview