My advice would be don't do it, not to waste your time, but neither Ralph (I assume) nor I have seen your original complaint letter to the BBC. This would have to be pretty watertight in terms of the objection that the article fell foul of editorial guidelines. Unlike some other complaints avenues, eg the Advertising Standards Authority, you need to get a final rejection from the BBC before you can go to the Ofcom. They are unlikely to consider new material, just look at your original complaint.
I suspect they will side with the BBC - articles do not have to be 100% factually correct, and how incorrect they may be is a bit iffy, because it is judged against target audience and impact. Further, Ofcom has no power to do more in the case of BBC web pages (as opposed to broadcast television) than issue recommendations. Frankly the chance of getting Ofcom to consider seriously an item on such (in the scale of things) a topic of minor importance is - I would judge - slender.
The problem is that the writer of the article will claim that she trusted her primary source of information.
I suspect the origin of her article was this -
https://blog.nature.org/science/2016/05 ... d-climate/
which cites this report in Nature
https://www.nature.com/articles/nplants2015142
And to be fair, the Kew research leader's comments do tend to support the "cacti fall victim to plant hunters" argument. Whether all this is news or opinion is, of course, debatable.
In comparison with this major issue the question of Alluaudia not being a cactus is fairly trivial (as is the whole reference to that source of info).
It is a pity there is no reference to contact details for Sylvia Smith, when maybe a more direct line of complaint might have some chance of getting an article to redress the balance.