Just to explain to people why these restrictions are in place:
Metal rails expand when they get hot. That's physics. If you welded them together at 10C and the temperature jumps to 30C, then the expansion will result in the rails wiggling as there's no room for them to expand, so they end up getting squished and they can buckle.
Engineers account for this by essentially stretching them when they're welded together. That way the heat will cause the rails to come back into their pre-stretched state instead of having them wiggle. If it's hotter than the pre-stretching compensated for then they'll still expand to the point of getting squished, alas the train can't go at full speed because it's dangerous to do so along rails that aren't straight.
As you might guess, the process of pre-stretching them costs money, and the price increases the more they're stretched. Someone somewhere crunched the numbers on what temperature it's worth spending money on dealing with (also accounting for winter lows) and that's how we have what we have.
Countries that have hotter summers will have also done these calculations and come up with a higher number, which is why you hear cries of "But the trains are running fine in Mexico and it's 40C!!!!", it's because it's common enough to spend money coping with it there but not here.
So yes it's annoying, but maybe knowing the reason will help a bit.
Just to pick up on the second half of this post. It doesn’t cost more money to ‘stretch’ (stress) the rails more.
When installing continuously welded rail, you have to pick a temperature at which there is no stress in the rails, and in this country we set that temperature at 27C - it is known as the Stress Free Temperature, or SFT. We could set it higher, but then that runs an increased risk of broken rails in winter - where rail temperatures can easily be below -10C.
Another point - rail temperature does not necessarily equal air temperature. The former is a function of air temperature, cloud cover, exposure of the rails to direct sun, and the angle of the sun (which is higher, and therefore more powerful, in the weeks around the summer solstice). It’s quite conceivable that we will get rail temperatures of nearly 60C tomorrow, and that will be close to route / region wide blanket speed restrictions. This is because it is impractical to impose potentially hundreds of emergency speed restrictions.
How they cope in other countries is to have a higher SFT. In the very few parts of the world with wider temperature ranges than the U.K. (eg the Russian steppes), they will restress the rails for a different SFT each spring and autumn. Or put up with buckled rails.