Coverage in Cities really difficult.

Does this happen in multiple places or just in one place e.g. home or office?

With large buildings in big cities, there are often 'dead' spots where signal does not penetrate. Worth checking if others in the building experience the same problem.

I also have this problem. Recently on a number of different days in Mayfair, Oxford Street and Tottenham Court Road, all in Central London - I was unable to connect to mobile data even outdoors. I had a full 4G signal, calls and texts were fine but no data. An exclamation mark appears on the phone signal indicator when this happens. It was very frustrating as I was trying to meet up with friends who were sending me WhatsApp messages to tell me where they were, and I could not receive them. Also I was unable to search on Google Maps. Admittedly I was in a very busy area but it does seem to be related to Three prioritising other connections over mine.

Yea, I have found that data doesn’t always work when on 4G, with signal. I think it’s congestion and hopefully Three get it sorted before they switch off 3G otherwise it will affect our ability to make and receive calls.

I have had this issue on my last 2 trips to London and also in Basingstoke, super annoying when trying to navigate using maps.

I also found myself in a restaurant and unable to buy lunch using their QR code ordering system which was a pain

Shane we can’t seem to post pictures any more. 

Did a Speedtest yesterday, as someone on the Bristol subreddit claimed that three / smarty was sh** everywhere in Bristol, so I did a Speedtest, got 639 mbps, and posted the video. Didn’t get a response 😂

I'v posted similar experience. Not just in London. 
I was recently at Westfield, stratford - two separate visits a week apart, at different times of the day - and the signal there was apalling. Either I got nothing at all, or when i did  get a few bars, there was no data, just voice.
And recently in central london - again, signal was patchy but data would cut out entirely.  I had to rely on customer wifi in nearly every situation.

Recently driving through Kent, despite having 2 or 3 bars on 4g, I got nothing data wise. It caused us no end of toruble - all 4 phones (3 iphones and 1 Android) could not get Google Maps or Waze and we were lost.

Weak signala nd no internet .

Now in our local town the signal is poor and again, no internet. THis was not the case a few months ago or last year when we switched.

I would conclude that THree network management is to blame, and if they re actively de-activating 3g matsts, then perhaps the remaining 4g and 5g infrastructure is seeing more gongestion as a result?

I am now actively looking at alternatives!

Like others here, I often find I have a signal showing but also the exclamation mark and effectively no signal.  This happens regularly (once a week) and in locations all over the UK.  Will probably mean I'll be leaving SARTY to go to a more reliable provider

Chalkychap  This issue is nothing to do with reception; it is a backhaul problem. My phone works fine in the City but as soon as I venture west, data slows down and eventually stops working around Cov G/Charing X area. I will have a full 4 bars of signal strength yet no apps work. This was never an issue with GiffGaff, which I will be transferring back to.

This is not a reception issue, it is a backhaul capacity issue. Smarty (or Three) doesn't have enough of it in many busy locations. My phone works fine in the City, however travel east to Cov G/Charing X area and, despite having a full 4 bars of signal strength, data simply stops working. It's very frustrating and I will be transferring back to GiffGaff, where this was never an issue previously.