Got fed up with intermittent Three/Smarty dead periods - maybe 3 or 4 a day, mostly in the evening, so have popped an unlimited 1pmobile Sim in my 5103e for now as an experiment.
The speed and signal is not quite as good as Three, but at least there are no halts to connectivity due to their core. I get around 130 - 170Mbps with b1, n28 and n78, whereas 250 - 400Mbps with Three on b28 and n78.
One thing I have noticed, and I think someone else here also pointed this out is that, when I do get these connectivity halts, if I'm on a VPN, such as with work laptop, I maintain connectivity throughout! So there must be some routing or traffic inspection fault somewhere in threes core rather than oversubscription.