2G uses the same bands as 3G/4G, and doesn't use a lot of bandwidth. It'll come down to cost versus benefit, and the cost of replacing tens of millions of legacy 2G IoT devices is going to be very high, and take a long time.
Every single smart meter installed in non-LRR parts of the UK will need to have its communications hub replaced. That's almost as big a job as fitting the things in the first place, and isn't going to happen overnight. It's taken a decade to get to where we are now with the smart meter rollout, and will take close to as long again to replace all those millions of communication hubs.
On top of that there are all the other IoT devices that need replacing as well, ranging from remote telemetry used by the utility companies right across the UK, to tens or hundreds of thousands of other sensors and controls used by everyone from wildlife and nature conservation bodies to trackers used on vehicles, etc.
All this stuff can be replaced with 4G compatible connectivity, but it will take a lot of time. Someone also has to foot the bill for it.