As for 2G/3G, virtually all Note 5 supports GSM 900/1800 UMTS2100 so that's not an issue. What we are concern with is 4G.
In UK, we uses 3 of the LTE bands: B20 800Mhz (02, EE, 3, Vodafone), B3 1800Mhz (EE, 3) and B7 2600Mhz (EE, Vodafone).
According to my research, SM-N920I supports:
LTE2100 (B1), LTE700 (B17), LTE850 (B5), LTE1700/2100 (B4), LTE1800 (B3), LTE2600 (B7), LTE1900 (B2), LTE800 (B18), LTE900 (B8), LTE700 (B12), LTE800 (B26), LTE800 (B19), TD-LTE2300 (B40), LTE700 (B28)
Although it supports 800Mhz, but it is B19 not B20.
B19 uses 875 - 890 Mhz as Downlink, 830 - 845 Mhz as Uplink, whereas
B20 uses 791 - 821 Mhz as Downlink, 832 - 862 Mhz as Uplink
So chances are, its not supported.
However, since it still supports B3 / B7. You can still pick up 4G from Vodafone / EE / 3 (just not O2) with EE as being the best option.
N920C, I'm having trouble to find the detailed specification. In one of the website during my search (findmeproducts.com), it says:
2G bands GSM 850 / 900 / 1800 / 1900 - SIM 1 & SIM 2
3G bands HSDPA 850 / 1900 / 2100
4G bands LTE band 1(2100), 2(1900), 3(1800), 4(1700/2100), 5(850), 7(2600), 8(900), 17(700), 20(800) - N920C
So my guess is: YES. However, you might want to seek further verification just to make sure it does support B3 B7 AND B20.