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.
Director of Brewing Operations
Thomas Hooker Brewing Company, Inc.
Bloomfield, CT 06002