Have a look at Alert ID - same company, same idea, do pet tags (which one could put on a horse) but (obviously because all horse stuff must have a premium added) - cheaper.
I do lots of lone working so I am looking into getting one, hence I know about the two. AFAIK they are a reputable company who have branched out into the horsey sphere.
I use them. I have received my registration pack (which cost me £16 as there was a special offer on, but is normally £25) with 1 x saddle tag, 4 x helmet badges and 1 x wrist band from them, and have uploaded all my details on to the web site. It was a very easy process.
Fortunately I have not had cause to use the service (touch wood that I won't ever have to!), but it all seems a very good idea to me!