how hardy is a paulownia tree in the u.k.

I have one and it grows every spring.

You can leave it to grow into a full blown tree which will produce foxglove like flowers when it is mature.
Or you can cut it right back down to the base, each spring where it will grow as a large shrub with huge leaves, or let it grow to about 6ft then chop the top off where it will grow each spring and produce huge dinner plate size leaves.

They will grow in pretty much any soil.

I have a couple that I prune back to get the huge leaves, and I gave one to my daughter, also living in the Midlands, and she let hers grow - and it flowered in 2008; there is a photo of it on my site. A lovely plant, go for it.

Paulownia will tolerate temperatures down about -5