It's your money, your shout.
Here's a map of the TrackR users around Great Malvern (assuming it's near you, going by the given location in your avatar).
View attachment 348549
I'd point out that their map of users in Isla Vista, "the theft capital of California" where they staged the bike theft demo, has a
much higher concentration of app users, yet it took 10 days for him to get a hit from a user who passed close enough to the bike to register its last known position.
That doesn't fill me with any confidence that it would be £25 well spent.
In any case, it's not an answer to bike thieves if it doesn't prevent them from stealing it in the first place, which TrackR can't do.