Identifying fault in a lighting circuit
It's your old dimmer, motion sensor, lighted switch, smart switch etc.

It isn't made to play with LEDs.  

A dimmer, smart switch etc. is an active machine which requires power to operate itself (i.e. always-hot and neutral).  Many old installations use what we Yanks call a "switch loop", containing always-hot, switched-hot, sometimes earth and never neutral.   (Our Codes now require provisioning of a neutral).  

Therefore older active switches (and some new ones) power themselves by "placing themselves in series" with the load.  When the load is off, they depend on a unique characteristic of incandescents that an unlit incandescent is a lot like a dead short.  The active switch   can leak small amounts of power through the incandescent without lighting it up.  

LED and CFL have switching power supplies which do not like that (unless they are specifically designed for that). When you removed the last incandescent, you placed the active switch and LEDs in combat with each other, hence the RFI.

You did not state which special ability the switch has, so it may not be important to you. Swap it for a regular switch and done.  Or, put one incandescent back in. Or get a modern LED-rated version of that thing and matching LEDs.