Hi Wayne,
Although there have been many claimed
sightings of this species in Britain during perhaps the last 100 years, I don't think any have been accepted by the rarities committees.
As far as trying to ID them on call alone, I recall an occasion in SE Europe when a friend and I tracked down what we believed to be a BW, only to find that it was two plantation trees knocking together in the breeze
They really are unmistakable once seen though.
If anywhere, you'd think they would turn up along the Kent coast (shortest sea crossing).
Can you describe the sound(s) you heard?