I haven't used cooling blankets. I have used ice packs before. They tend to be a bit messy. Lots of condensation dripping off of the cask.
I have a lot of experience with the cooling saddle at the top of that same UK Brewing page. They work great. Do you have a draft glycol loop in the area where you are planning on cellaring your firkin? If so, you can tap into that line to run your saddle.
You can make a saddle out of copper tubing to test its viability in your environment. The copper won't last forever, but it will certainly let you see if it will work.