Grid frequency is maintained in accordance with the Grid Code which all of the UK Generation companies adhere to. This sets grid frequency at 50Hz +/- 1%. The relevant link is posted below. I have spent most of my working life (>30 years) within power station control rooms and my own experience is that there is more volatility in grid frequency now, although this volatility is still within the 1% tolerance. The household voltage requirements are set (I believe) by Electricity Safety, Quality & Continuity Regs (ESQCR) and are 230V +10%/ -6%.
There are many factors which influence your household supply, one of the more significant being the distance that you are from your local substation and whether there are any other "large" consumers fed from the same distribution line.
CCGT generation (gas generators), wind farms, hydro and interconnected supplies to France, Ireland and the Netherlands can and are all used to balance and ensure Grid stability. I don't agree that modern generators are less able to "modulate", quite the opposite in my experience, I was very impressed by the speed at which modern CCGT sets can be spun up, synchronised and loaded. Modern forecasting techniques also now enable renewables to contribute in a much more controlled fashion than even just a decade ago.
https://www.nationalgrid.com/sites/d...rid%20Code.pdf
This second link allows access to see grid loading in real time.
http://www.gridwatch.templar.co.uk/