Its not the band itself that drives the potential capacity, its how
much of the band the provider owns.
Vodafone (and O2) own 3x as much (15mhz slice) as either Three or EE (5mhz slice each) in Band 20 (800mhz).
Theoretical download speeds for Vodafone B20 are 112.5Mbps, whereas its 37.5Mbps for Three/EE.
Vodafone use B20 as as their primary 4G carrier so is very widely deployed on lots/all masts, unlike Three and EE who really only have it on certain (tactical) sites.
As such, the site density for Vodafone B20 is much higher than Three/EE's B20 site density - as a user of B20 on Three/EE you'd be getting a much smaller share of capacity from a mast thats likely a fair distance away than if you were a Vodafone B20 user with a mast closer to you.
Conversely, Vodafone and O2 both only own 5Mhz slice in B3 and use it for 4G, where Three own 15Mhz and use it for 4G. EE own 45Mhz and deploy up to 40Mhz of that for 4G use.
Peter has the spectrum allocations shown here:
Commercial mobile spectrum use in the UK
pedroc.co.uk