Have just found this:
http://www.toolstation.com/search.html?searchstr=16578
but the max load is 400W and I need to control 600W.

Jesus. I hope you're responsible for floodlighting the Eiffel tower or
something. 600W is usually utterly inappropriate even for the most obscene
interior or exterior lighting in a domestic property. Perhaps you should
work on reducing the load, rather than finding a larger switch?

Apart from that, you'll need some sort of immersion timer with the right
features and 13A ability. Alternatively, an external relay.

Christian.