I'm a long-time fan of the Frost series, but this episode just didn't agree with me.
My main complaint (as actor David Jason himself agreed with at the close of the series) is that Frost is simply getting to old to be going after younger women, in this case a lithe aerobics instructor. Frost is now easily into his 60s, while the instructor looks be be no older than late 30s. In earlier episodes, Frost has occasionally had more than a protective, parental eye for the ladies (including ex-madams with hearts of gold), but this is definitely pushing things.
Another criticism: rare species trafficking is and was a serious problem, but it should have been addressed as a background crime issue, not the continuous public service announcement as it appears in this episode. In previous installments, the series adeptly handled more serious (even heartbreaking) issues like child sexual abuse and endangerment without becoming maudlin or preachy in their treatment.
Lastly, having conclusively identified the real mastermind behind the trafficking and murders, Frost normally would have read him his rights, cuffed him and had him hauled off straightaway. Instead, he chats at length with the criminal and allows himself to be led into a situation involving a dangerous wild animal. In the ensuing showdown, it is all too obvious that Jason's stuntman is doing the wild-animal taming act.