Will I be able to get 5G anywhere in the UK?
No. In the future it may be possible, but right now, like 4G, there some blackspots where reception is patchy at best or not there at all. See where you can get 4G currently. That won't be solved entirely for some time. As you probably know, 4G can come and go even in big cities.
The good news is that 5G doesn't slow down even if there are loads of people using the same app in the same place, thanks to its improved capacity. At the moment, even with 4G you may experience a lag or a decrease in service. With 5G, the reliability is so superior that experts believe it will be trusted for all sorts of initiatives, like autonomous cars and virtual reality medical treatment.