Keep in mind that current 5G deployments in the UK (and I think the world) bond 4G and 5G together. This often causes problems if the 4G is heavily congested. Vodafone's London 5G deployment is still pretty spotty, it will get a lot better. Plus it uses a very high frequency band (3.5GHz). When the 700MHz band gets added in a couple of years it will be a lot more consistent.
Yes, SDL is on LTE1500, LTE Band 32. It is another carrier to aggregate with the other ones. Both 3 and Vodafone have some spectrum in that band, IIRC 3 has a lot more. It will really help with overloaded cells in the short term.
My router (Huawei E6878-870) apparently supports LTE band 32, but I've never seen it used on either Three or Vodafone. At my flat, Three seems to use band 3 (1800 Mhz) always.
Vodafone floats between bands 7 (20 Mhz channel!), 20, and 1, but gets good speeds on any of them.
Three always slow at peak times, sometimes unusably slow, despite having by far the strongest signal.
Yes, SDL is on LTE1500, LTE Band 32. It is another carrier to aggregate with the other ones. Both 3 and Vodafone have some spectrum in that band, IIRC 3 has a lot more. It will really help with overloaded cells in the short term.