In the plant case, among many other interesting things (fungal interactions, say) I think of examples like: How roots grow towards water/nutrients (even if rocks are in the way). How leaves/branches can lean-or-orient-towards and grow-towards sunlight. The kind of self-healing that this article describes, and the overall way in which everything a plant does tends to lead towards it being able to reproduce via seeds or spores (or one of the many other ways that plants can reproduce!).
I'll freely admit my perspective on this is influenced a lot by Michael Levin's work around hierarchies of agency (many vids on YouTube) In many of his talks he describes how agency can be treated as a measurable quantity, like something from engineering. This places it far from philosophical or abstract definitions and more in a cybernetic realm where agency is a measurable thing and can either be instructed from outside (like a thermostat) or from within - like when a hungry animal (or plant) seeks food - and as you go up the scale-level for agency you get entities that can legitimately have (and achieve) clear longer-term (and spatially larger) goals. Smart animals - such as humans - can even with a mostly-straight-face talk about and put into place plans that would reach far into the future compared to their own expected lifetime.
So yeah I agree plants certainly can't comprehend BBC2, but I think if that's the definition of agency then we're really not talking about the same thing.
I agree that plants worry about light and water, but they don't just sit there and do nothing! They respond to it with their agency, maybe it's an agency we find hard to recognize and don't fully appreciate and we'll often say it is small/slow or disregard it, but I personally think that plant agency is made of the same stuff as human agency (and we just have a special word for it when it's us: consciousness).