After further testing the lights do not flicker but then do dim, is this normal when powering on the hoover? And slightly but not as much when clicking on the kettle.
Is you mains power supply fed to you by overhead wires? Are you at the remote end of a country lane, in the village of "Much Mucking In The Duck Pond"?
If both answers are "No", then I would not expect that powering on the vacuum cleaner or the kettle would cause the lights to dim, transiently.
I think that Black Sheep and / or Walter may be the best qualified to comment on your power situation.
As with all things, trying to 'bottom out' an issue on a forum is difficult. I wonder if we're going off at a tangent as well here ?? Anyhows, as most folk will know, the lighting circuits and power circuits (ie: ring main sockets) should be on their own protective device, be that a rewireable fues, a cartridge fuse, or an RCD/MCCB etc etc.
However, there could be a fused-spur feeding a lighting circuit of its own, that is fed off the ring-main wiring. This is quite legal to do. It is most commonly used in loft-conversions or conservatory builds, whereby it's generally easier to extend the ring main into the 'new build', but trying to pick up the lighting circuit proves more difficult. By running a seperate 'fused-spur' that is teed-off this extended ring-main, you can fuse the circuit down to 5amps and run a lighting circuit from it. In effect, the 'fused spur' is acting as the main distribution board, but only for that particular lighting circuit. So, it is possible that if the ring-main (with the fused spur attatched) is working at near full capacity, then switching on a bank of lights could see a small voltage dip.
Hope that makes sense, cos it's not easy to put over in words.