The advice from Andy FMF is good, take a second SIM ideally in its own handset. That can improve your chances of coverage. A Manx Telecom sim is ideal as will roam on to all networks, they are used in the Highlands by telco’s and utilities who cant justify Iridium.
If you just want to take a second prepay sim, select it like this
Main sim = o2, Voda, Tesco, Asda take an EE sim
Main sim = EE, 3, Virgin, BT take a Voda sim
This is derived from the way the networks share masts and infrastructure, for example Voda and o2 share a lot of masts so their coverage very similar so little point in taking both.
Being able to switch to 2G only, 3G only and 4G in your settings can somtimes help near a summit when you have signal, but cant make a call / move data. That’s typically when you have too many sites in range or possibly the serving cell on 2G is more than 35Km away. But normally leave it set to 2G/3G/4G auto.
Good luck and enjoy your trip