There's a fair bit of incorrect/misinformation here...
Where are you getting this from?
MBNL throws users to 800 B20 when it can on both its (partner networks.
Vodafone/o2 (Cornerstone partnership) don't display the same behaviour anywhere like MBNL.
MBNL and Cornerstone agreements are not comparable. MBNL is a RAN (effectively just the antenna) share agreement for 4G. Each provider runs their own baseband radios, backhaul (mostly...microwave link might be shared) and power supply.
Cornerstone is an agreement whereby one of the two providers build/install/manage the an entire site for both providers. This is why in their own 'host' areas the providers are generally much better equipped - they can do things more easily/quickly than they can in their non-host areas.
Post BT EE purchase Three was as allowed to build outside MBNL as BT had substantial contracts for MBNL backhaul pipes, that also went both ways & EE appointed Nokia Solutions and Networks to build it’s own outside MBNL.
Three have always been able to build outside of MBNL if they wished - there was nothing stopping them - it was more that they were biding their time with their MBNL 4G sites (which are generally well positioned and dense enough to give adequate coverage as EE and Three both originally used B3 for 4G), adding more 4G only would have made no sense as 5G was just a few years out. BT buying EE has nothing to do with this.
Shared hardware broadcasting on MBNL sites replicate the same on EE & Three, it’s not just simply passing saturation off, as I stated priority is given to the MNO then (even internal) MVNO’s, then add in any internal of Three/EE programming.
As above, each provider run their own network, the configurations applied at every site for each provider will be different to one another. There is no prioritisation (CQI) or band steering based on user/SIM/account type happening in the
radio part of the networks (though I cannot comment on things like QOS rules further upstream, in the providers cores (for example, smarty's data goes through a proxy, which may slow things down)).
Three contract sims in no way hands off to B20 like EE contracts (‘Essentials‘ & now rebranded ‘4G only’ does).
Again, as above, each provider has different configurations. Three
really don't want you on B20 and have the reselection metrics configured as such (so you'll very very rarely see it - in fact you'll drop to 3G before B20). EE seemingly don't mind so much and have less strict configurations, so you can drop onto it more easily (though it's generally less widely deployed on EE than Three so you may not see it anyway).
Three remains consistent with even minimal B20 handover even on 4x4 CA device. EE will by default on a non Max/5G to B20 and 8 Mbps on the same, lockdown to high speed 2600MHz and it flies with some minor blips whilst EE tries to force across to B20 that the device says sorry pet not available, I’m sure if EE 4G their 700MHz aswell as 5G it will get worse.
Band reselection is doing its thing, either through load balancing due to overloaded higher frequencies, or the metrics of those higher frequencies fall outside of EE's configuration and so you're reselected to a lower frequency where the metrics are within their bounds.
EE aren't going to use 700Mhz for 4G, only 5G.
I’ve experienced the exact same behaviours city wide. EE loves band 20, it’s the difference between HD & 4K streaming, saving them money.
Business 5G sim worked flawlessly
Never a issue keeping 4K Three streaming regularly.
Plan/Band control is how to get around Net Neutrality.
T-Mobile had such in place for 3G Web n Walk and that’s the 4G system EE is built on, T-Mobile! People had to get T-Mobile HSDPA switched manually on their alleged HSDPA plan. Also now known at EE (4G/LTE) as the £2 “Go Faster add-on”.
Why are you comparing 5G to 4G? You could have the weakest of B20 4G signal but if you had good connection from a nearby ENDC enabled 5G site then of cause your throughput will be boosted by the 5G bearer.
Band reselection is not 'getting around' net neutrality - it's literally how radio technologies work with physics to provide the best signal to a given device in a given place with a given set of radio conditions.
It's like saying your 2.4Ghz WiFi is 'getting around' net neutrality because it's giving you lower throughput than your 5Ghz WiFi.