Do you still pick up an orange signal?
This is what I get on my T-Mobile contract SIM only (3g) phone, in the same place I had very good 4G yesterday:
Firstly you don't have the newest carrier settings as it should show EE and EE(3G).
In your case, as you're on T-mobile...
The T-mobile would show as EE(3G)
The T-mobile orange would show as EE.
Even if both were actually 3G. It displays the home network first as EE(3G).
Now, in time you will still find T-mobile Orange - but if you try to connect to it, so try clicking on it, you will see No service. Eventually the network T-mobile Orange will go completely leaving just T-mobile or EE(3G) on T-mobile.
If you can see T-mobile Orange and connect to it, it indicates an orange mast is in place and was kept there as the orange site is going to be 4G enabled. When the LTE upgrade takes place, at first you will still see the network but there will be no service, and with time you will not see the network at all.
In parts of Westminster you struggle to find an orange signal and where you do it's not connectable. In time it will go entirely.
This is good because currently to switch from an orange signal to a t-mobile signal, the connection has to physically drop. So say you're making a call on an Orange signal and go into an area with only T-mobile. As it stands your call would drop out whilst you physically switched networks. Once everything goes through there's going to be LTE and T-mobile type 3G and 2G only, so it'll be one network with no messy switching, meaning hopefully less dropped calls and a unified signal.
This is why EE is better than 3 signal wise (overall) because they kept Orange sites as well, in places MBNL sites didn't reach too well.
Some orange devices seem to be sticking to Orange signal & holding on too long before switching to a T-mobile signal. As orange sites are decommissioned or replaced with LTE this causes them to have bad signal when there's a brilliant T-mobile signal available. There's nothing that EE can really do about this as it's the device at fault. People using older carrier settings find issues as going onto T-mobile makes the device believe it's roaming onto a foreign network, so it's less likely to happen, their devices hold onto a weak orange signal before doing it... Those with newer EE carrier settings, their devices no longer see the other network as roaming and this somewhat solves the issue. The combination of the EE carrier settings and the smart-sharing works somewhat well but it's not perfect.
EE in time will have one network and no orange signal, at this point problem devices will be forced onto T-mobile as there will not even be a weak orange signal available. At this stage all problematic devices will be fine, although decommissioning of orange sites may cause some people to lose service entirely but that's not a large chunk by any means.