I'm in Sheffield and have been experiencing intermittent connectivity with Smarty's (3 MVNO) since around the 6th of December, with it frequently cutting out every 15 minutes or so. Been using 4G, though the same issue persists on 3G also. My signal strength is perfect, and I've not had any issues with the service until recently, so I wouldn't think its anything on my end.
Web chat don't seem to be aware of any ongoing technical issues, and just provide useless template responses as expected.
Still, good to know it's not just me - I was worried they were restricting the SIM for using over a TB each month.
SMARTY 4G (Three) — 60 Mb/s down/30 Mb/s up