There probably isn’t much on iNaturalist that would show you a British wood, given that we are discouraged from posting habitat pictures. And the line between natural woodland and plantation is quite blurred: there is no virgin forest here. All the ancient woodland will have been used over the centuries with bouts of replanting because very few people here believe trees are capable of natural regeneration.
By the way, ancient woodland has a particular meaning in Britain: it is an area that has been continually woodland since at east 1600. It may have been felled, but has not been put to a different land use.
You also have to be aware the word forest has three meanings: the romantic meaning of a large tract of wild wood; the Forestry Authority uses Forest to make its plantations sound environmentally friendly, e.g Newborough Forest, a huge conifer plantation on what was Europe’s biggest sand dune system; and Forest was also the term for a tract of land reserved for royalty to hunt in, often dating back to the Norman invasion, 11th century, and this type of forest will often be a mixture of vegetation types, e.g. The New Forest contains heath, grassland, bog and ancient woodland (as well as conifer plantations).
So bearing in mind those caveats, try googling Monks Wood, Epping Forest, Black Wood of Rannoch, or just UK ancient woodland.
If we acknowledge that “natural” ecosystems are not the only important ones, then there are certainly beneficial invasives. For instance, what keeps us humans from overrunning and using up the remaining “natural” ecosystems? Answer: agroecosystems. Invasive earthworms may be a bad thing in the North Woods, but in the farmlands adjacent to the North Woods, they are a good thing, helping to keep the soil friable.
Right there is some nuance here; what about cases when an introduced species is just filling a niche that only exists because of prior functional extinctions? For an example out here in the west, cattle grazing is often derided as harmful but it is just filling the ecological niche that now-functionally-extinct Bison should be filling. I am pretty certain that it would be super ecologically damaging to ban cattle grazing without also simultaneously reintroducing Bison; everything native out here is grazing adapted, but many invasives are not! It is sometimes blindingly obvious to look at a fence running through a field that is grazed by cattle on one side and not on the other; on the cattle-grazed side it can be all the delicate grazing-dependent native flowers, and on the ungrazed side just grass and noxious invasives.