Last Legion in Britain???

[Robert Vermaat]

What do you think about roman legion sent in Britain later (it was mentioned by Bede, Gilda and some annals)?

That's from Gildas, and Bede is merely quoting Gildas (and therefore not an independent source). Gildas calls all units 'legions', and since we know that since Diocletian, newly created units were also called legions, this need not mean any of the two classic legions.
Personally though I think Gildas is stylizing earlier history into three 'expeditions', as he also has the Romans invade Britain three times at the start of Roman history in Britain.