Are you sure it was Acacia you saw and not Cercis? Acacia baileyana 'purpurea' might be it otherwise, but they're only frost hardy, though in a very sheltered spot in the south, maybe they would survive long enough to flower... not sure if the heat last year has anything to do with it though.

Oops I think it might have been Jacaranda - I only saw them from the car.

I doubt its Jacaranda either - that's a tropical/sub tropical plant. Perhaps it was Cercis, although the flowers on that are rather more pink than you're describing...

Could be Paulownia tomentosa. Look it up and see if you recognise it.

You've got me thoroughly confused now! I saw the same trees in Spain a few years ago and they were definitely more on the pale purple blue side. I really wasn't near enough to see but I might pop into the house near me that has it and ask. Thanks everyone.

It couldn't have been a Ceanothus, could it? They can get small tree sized....

It was quite a big tree. Saw it again today on the motorway up to Bucks. I'll post when I have asked the owner near to me.