not sure you can get the Marineland Visitherm Stealth heater in the UK, but what i was advised to buy is a hydor theo heater. they don't have a traditional coil element in them, carbon block or something instead i think? but apparently they will cope with being left out of water for a while. although its still not really recommended.
or if you have an external filter there are external inline heaters which connect onto the outlet pipe of the filter and they state that they can be run dry. i plan to get some of these in the future when funds allow.