Although initially they look similar, there are several obvious differences between them:
Stalks: The native bluebell has a curved stalk, whilst the spanish one is straight.
Flowers: On the native bluebell, the flowers are on one side of the stalk, whilst the spanish ones have flowers on both sides of the stalk
Colour: The native bluebell is a darker shade of blue
Scent: Native bluebells produce a scent, whilst the spanish bluebells don't.
The difference can be seen when comparing a bluebell growing along the ock valley walk to one growing at Wytham Woods:
The problem is not the spanish bluebell replacing or out competing the native ones but the cross pollination resulting hybridizing.
As the bluebells in our garden have traits of both bluebells, they are probably hybrids.