I am likely showing my zone 7-8 plant ignorance, but, I thought zone 7 and above was fine for Jacaranda. Maybe you don't have enough intense sun? I have always thought the UK was all zone 7 and warmer. Only the most severe would be zone 6, but, I really thought that zone 7 was the coldest zone in the UK. I would be shocked if there was a zone 6 in the UK. I have been in the UK in March and it is chilly, to damn cold, but, I had seen all foliated trees when there. I was there in spring 2001, so that could have been a warmer year than some afterwards. I wish I had the money to go back to the UK. I would love seeing some of the old "dodders" in Winsor forest, you know those 1,000 year old Q, Robur trees, especially those with trunks so large you can have a bar or home inside. I would love to see the ole Q. Ilex at Cardiff castle. My friend took a pic of me sitting upon it's rootmass, many years ago. I would love to see the old specimens there, which i was totally unaware of. Of course, when I went, I was not aware of the impressive trees that were there. I had actually been impressed with the castles that were there to see.I am happy to say, my friend and I saw many great castles on our trip. I had more fun on that trip than ever in my life. I will post thre pic of me on a Q. Ilex rootmound back in 2001, in March. {{gwi:476766}}

The winter cold isn't the problem, but rather the summer cold. But if the jacaranda is planted up against a south facing wall the chances for survival will increase.

Poaky - I know that tree. A lovely thing.

I have never seen a Jacaranda growing outdoors anywhere in the UK. The US zoning system is pretty meaningless over here as it refers to winter cold, as Huggorm said. Our Winters are relatively mild but our summers are relatively cool. Many trees , such as Crape Myrtles, which can be grown in places in the US with much colder winters than ours do not thrive here because there is insufficient summer heat to ripen the wood. Even ROS is not hardy everywhere.

Mushibu - there is a useful bit of information on this old thread. Scroll down to the input from DavePoole.

Here is a link that might be useful: Jacaranda

I wouldn't risk planting it, may place outside but I don't know.
Thank you floral_UK I'll have a look now.