Take GREAT care - I can't get any signal indoors because theres no 800mhz in my area. Check on Ofcom's coverage map before you trust Virgin/EE's. Or look at https://opensignal.com/ which may give you an even more detailed map.
Also form EE. To avoid confusion, EE has also ‘blocked’ phones that cannot run with VoLTE from detecting the 800MHz frequency, to avoid any confusion caused by bars displaying on the handset, while being unable to make phone calls.