Don't bother unless it's £3k or less. Those prices are money for old rope. 3 year old specs.
You can get a top 2019 iMac for £3600. So why bother getting a refurb iMac Pro for more(?) for anything other than it's more extensive cooling.
When the new iMacs launch (imminent) they'll put the current iMac and the entry iMac Pro in the shade. Probably in performance and price. Saving thousands in the process.
Azrael.
It has much better thermal cooling, better sustained performance, 2 additional TB 3 ports, 1080p heavily improved FaceTime camera, mic-array setup, improved speakers, to name a few more changes. I would definitely go for the base iMac Pro over an upgraded 2019 iMac.