Sites that support the use of CGI or other digital manipulation effects do not provide supporting evidence. Specifically, what you see is really happening, whether in nature or in the lab. However, scenes placed together may not have actually happened together.
Say that documentarians want to capture a kangaroo fight. Getting this kind of footage can take a long time — maybe days or weeks of filming kangaroos doing normal, boring kangaroo things. During that time, filmmakers can get a bunch of footage that can complement the eventual fight: a joey hiding in a mother’s pouch, close-ups of two kangaroos looking at each other, and so on. Then they combine this footage to make a much more emotional scene — one that tells a story with characters.
Furthermore, sounds accompanying any part of the documentary may not have been filmed with the image. The BBC does not deny cutting/splicing scenes or adding sound to its documentaries.
To confirm this, I contacted the BBC. Their response:
Our wildlife and nature programmes do not involve CGI. Where CGI and suchlike is used, for example in our "Walking With Dinosaurs" series, it's made clear this is obviously the case.
For "Blue Planet II" specifically, as we explain on our webpage at www.bbc.co.uk/news/entertainment-arts-41740841, the overwhelming majority of Blue Planet II has been filmed in the wild with just a few sequences created under controlled laboratory conditions where very specialist filming and lighting equipment was required to capture the animals and activity being explained.
In other words, CGI was not used (note that the linked BBC article mentions filming close-up scenes in a lab). Regarding the iguana chase scene specifically, the BBC has this to say.
The BBC has denied claims award-winning series Planet Earth II faked a nail-biting scene showing a baby iguana being chased by racer snakes.
To answer your question:
Is there any reliable evidence that the BBC has used any CGI or image composition in the snake-iguana chase scene of Planet Earth 2?
Ultimately, the BBC's goal is to "represent nature the best we can", which may or may not include filming in a laboratory. Nevertheless: No, there is not reliable evidence that the BBC has used CGI "or suchlike" in any scene of Planet Earth 2. In fact, the BBC has explicitly denied this, and continues to do so.
Note: While this question dates back to 2016, it may still be relevant as Planet Earth III is currently in development and coming to screens in 2022.