Dimmer for electric underfloor heating element
How can I create a dimmer which will work with 6A?

It's probably not a great idea to use any type of light dimmer topology because they switch the AC waveform on with fast edges and, given that your load is a distributed series of heating elements (i.e. it has a large magnetic loop-area), you might cause high-frequency disturbances that cause radios not to work i.e. they'll pick up a lot of interference (and maybe your neighbours radio as well).
The thermal inertia of your heating system will be large so, instead, use a timer type circuit that switches the AC on for a minute then off for a minute etc.. You should be able to find some circuits that do this based on a 555 timer and an AC relay.