Welcome back to the forum wavejockglw. I see you might be referring to 3 again in your previous post as you refer to networks with large numbers of dongle users and offering 3G to compete with fixed line - anyone who knows mobile will read from that you are inferring the Three network.
With the right capacity I don't see that becoming an issue, as usage increases technology catches up. HSPA+ 21Mbps, HSPA+ 42Mbps, LTE advanced...
Surely the answer is more lower powered base stations? If you have low capacity you divide your cell area up, reduce the power and use more cells over a smaller area, if necessary microcells and picocells. You can re-use frequencies as you go, only on a smaller instead of larger scale.
The Register had a good article about this strategy, showing that the future of capacity for mobile networks is not put it on top of a hill and turn it up full, but more localised lower powered coverage. Mobile technology will grow and grow with more people wanting to use services on the move.
In the US AT&T are delivering Netflix (which is 50% of US internet traffic) without issues using HSPA+ and LTE, the same with Verizon. If you look at US mobile reviewers on Youtube they test these things in busy cities like San Francisco and LA working perfectly fine. The speed tests are coming out north of 10Mbps in most cases too, but then the US are ahead of us with mobile technology at the moment.
Streaming doesn't use a lot of bandwidth, often it's less then 1Mbps, so if your network is upgrading to keep up with capacity and Three seem to be doing this then I don't think there is an issue. I certainly wouldn't be as bold as to say none of the UK networks are designed to cope with widespread streaming as they do very well coping with the demand today and have future plans to meet capacity needs.