Bluebell

How could one not include the Bluebell? There is something quintessentially English about this plant, even though it is found throughout the British Isles (except Orkney and Shetland). In Scotland a Bluebell refers to a Harebell here; they call it a Hyacinth. It's Latin binomial is Hyacinthoides non-scripta, previously Endymion non-scriptus.
So how do you tell a native from an introduced Spanish one? 
Well the key features are its form: the native has a one-sided raceme of dropping flowers with long thin bell-shaped corollas. The Spanish version has the flowers more upright and coming out spirally all around the flower-stalk. 
The other clincher is the stamens: in the native the anthers are pale cream whereas they are blue in the Spanish. Also the filaments (the 'stalks' that the anthers are on) of the three outer petals are fused to the petal all the way along apart from near the end (see bottom left), whereas the Spanish plant has filaments fused only as far as a third the way along.
In reality, the Spanish bluebell is rare. The far commoner garden Bluebell (and therefore garden escape) is the Hybrid Bluebell, H. x massartiana, but that's another story.