The law is the law, but if the brickwork of a chimney is in good condition, the additional wind load imposed on it by an antenna, adding perhaps 5% to the total area, will unlikely be the cause of failure. If a stack is in poor condition, it won't need an arial to get it to fall into bed with you!

A plain chimney pot would impose far more stress - [ it's amazing how big a chimerney-pot is when you climb up and get close!]

Here's a big ol yankee pot, up close! cool

http://img2.timeinc.net/toh/i/g/0208_chimneypots/installing.jpg

And here's some Victorian monsters from England, nigh on 5 feet tall!

http://www.brightonfireplaces.com/images/two-large-chimeney-pots.jpg










Wood work but can't!