Range from the cheapest Downlights installation £260 (London 24/7 Electrical Ltd based in SE26) to the dearest Downlights installation £400 (Elmark Ltd based in SE20).
Comparing electricians estimates for Downlights installation gives you an idea of who will be better value but the best way to compare costs is to get actual quotes.
What to do - Get rates and quotes for installing downlights by posting a job to electricians in SE26.

" Removal of existing light fixture Installation of: 6-8 Downlights A pendant ceiling light bar with 3 hanging cords and light bulbs (already purchased) - located above the kitchen island in the kitchen. Would also like the kitchen switch to be upgraded to a double dimmer switch. ..."
BR3