Coverage in Cities really difficult.

The 4 main mobile network operators in the UK use different parts of the radio spectrum allocated for mobile networks. Some frequencies penetrate buildings better than others, and the proximity of the signal source has a bearing on reception as well.

At any given location, there's no guarantee that each of mobile network operators will deliver the same signal strength, which maybe why your friends aren't affected to the same extent as you, Dominic_1770810.

Three UK (SMARTY Mobile's parent and owner) have an active bandwidth management policy. Three say it's designed to ensure whenever there's congestion or network issues, they'll manipulate their mobile data network in order keep things running as smoothly as possible. However, if bandwidth is limited and there are many, many concurrent users, things will slow down, despite active network management.