7.5kW draw to charge an EV is actually
less than most domestic electric showers in the UK (9 kW or so usually - 40-45A draw, more tna teh nominal '32A' EV domestic charger)
Such a large voltage drop as the OP is seeing, therefore looks to me to be worth following-up on; something is
not right.
ON mains frequency - yeah, well, being a measurist-weirdo I actually have a frequency meter hooked-up and have been watching; on the UK mains you get v short term changes in the 0.05Hz range, and total deviation - over last three months is typ less than +/-0.1Hz in say about a 3-5min window, and rarely worse than that. It is
observably, very much more stable in terms of short term frequency deviations on days when wind output is low, and remaining traditional (Nuclear/CCGT generation, with large spinning mass) is delivering >50%.
eta: mains voltage/freq at time of post: that's 49.xxx Hz