There is a technical reason for that, 5G in UK (and 99% of the world) is NSA, ie not stand alone, it acts as a supplementary band, anchored in LTE. Your phone/router will only aggregate this 5G band under load (ie downloading, streaming hq etc), most of the time there is little load so 5G is not needed. It is also a power optimization issue, as 5G processing requires more battery.