No the main culprit is older estates (both residential, commercial and industrial) which have no way to separate foul water from surface water and have no way of attenuating storm flows on site. From older sites it all just flows together to the treatment works. Then if course you have the people building extensions who just tap into the nearest sewer.
It was probably late 90's when the idea of attenuating surface water on site and releasing at a controlled rate became accepted practice. Although again older sites only attenuated for a 1:30 year design period.
Modern developments attenuate for storm periods up to 1:100 year plus 50% climate change allowance plus a 10% allowance for urban creep. They store far more water on site. The downside of course is the need for either large under ground tanks or larger areas of land take for above ground storage.
In theory if the water authority is aware of ground water entering their foul sewers through broken joints or pipes then they should repair them. Of course knowing that the water is entering the pipes is another matter.
The surface water from the development where we live which was started in 2019 drains into the open attenuation basin before being released at a controlled rate into the adjacent watercourse via a flow control device. Added bonus we don't pay Severn Trent a surface water treatment charge as part of our bill.