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I am the author of the instructions; I'd say that 25W / 2.09A is a bit on the high side for an 8" SCT. Using the calculator the heater I built is more like 19.5W / 1.65A.
That is perfectly adequate for UK conditions - it keeps the corrector dew free all night without a dew shield. I insulated the outside of the band with some rubber window draftproofing strip. It is not particularly thick so you might well get away with a lower power output if you have better insulation and/or a dew shield as well. Bear in mind you're just trying to keep the corrector above the dew point, not heat it so it is warm to the touch.
For my ED80 I actually wrap the same band around the outside of the metal dew-shield (goes around twice) and set the dew controller to roughly 1/3 to 1/2 power. Radiating the heat from the metal shield is actually more efficient than trying to conduct it through the front ring and corrector in the SCT. Your mileage will vary depending on your local climate of course.
240V definitely presents hazards in outside - make sure you use an RCD and have appropriate fuses in the right places to protect the equipment. I really wouldn't try to run a cheap eBay supply at maximum rating full time - either buy a better supply or use more than one (but read up about ground loops in either case which can be a hazard).
12V batteries are not as safe as you'd imagine though. No danger of electrocution, but there is a real risk of fire in case of a short-circuit. With a power supply a fuse or other component will blow if you get a short in the band or elsewhere, whereas with a battery you will get al the amps you need to cook your scope or fingers, so make sure you put a fuse inline between the battery and the load.
Hope this helps and feel free to ask any more questions you like.
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