We are encouraged by well-meaning people not to offer advice on matters electrical - danger lurks in bad advice.
There is only one thing that will cause the lights to dim and that's a drop in the voltage being presented to the lamp.
The AC mains supply to a property in the European Union is 230 V ± 10% at 50 Hz and your kettle, saw or router should not be able to cause a noticeable voltage drop when switched on.
The fault you have very probably lies in the area of the distribution board and the services of a certificated specialist will be safer as will a more modern consumer unit with protection devices.
If you're concerned about the suggested cost then perhaps you could seek a second opinion/quotation.
Take care...Good luck...
Peccavi
August 2009