Not sure why jigger is suggesting X370 
@Kam_s do you do any gaming, I guess not and solely just video editing? So do you not need a new GPU, does any of your software have GPU hardware acceleration that would benefit from a GPU? If not I guess you could re-use your fairly old GPU still.
I would start a new build based with something like the below maybe. The mobo has 8x SATA ports, best you can do I think on AM4? So I'd just get a 4 or 6 port SATA card for £20-30 I believe they are. NVMe for your boot drive (Windows and your apps etc)...
An alternative case I thought of after actually, is this Phanteks Enthoo Pro 2 which is new out I think. Don't think it comes with any fans preinstalled, so you'd need a pack of those and it is quite big... But looks like should hold a lot more HDDs?
You could also up CPU to 3950X if you really think need the CPU power. But looking at a few render benchmarks the 3950X doesn't seem to things that much quicker than the 3900X to warrant an extra £260? Up to you to decide if worth it though.
My basket at Overclockers UK:

@Kam_s do you do any gaming, I guess not and solely just video editing? So do you not need a new GPU, does any of your software have GPU hardware acceleration that would benefit from a GPU? If not I guess you could re-use your fairly old GPU still.

I would start a new build based with something like the below maybe. The mobo has 8x SATA ports, best you can do I think on AM4? So I'd just get a 4 or 6 port SATA card for £20-30 I believe they are. NVMe for your boot drive (Windows and your apps etc)...
An alternative case I thought of after actually, is this Phanteks Enthoo Pro 2 which is new out I think. Don't think it comes with any fans preinstalled, so you'd need a pack of those and it is quite big... But looks like should hold a lot more HDDs?
You could also up CPU to 3950X if you really think need the CPU power. But looking at a few render benchmarks the 3950X doesn't seem to things that much quicker than the 3900X to warrant an extra £260? Up to you to decide if worth it though.
My basket at Overclockers UK:
- 1 x AMD Ryzen 9 3900X Twelve Core 4.6GHz (Socket AM4) Processor - Retail= £439.99
- 1 x Asus TUF Gaming X570-Plus (AMD AM4) DDR4 X570 Chipset ATX Motherboard= £199.99
- 1 x Corsair Vengeance RGB PRO Black 32GB (2x16GB) 3600MHz AMD Ryzen Tuned DDR4 Memory Dual Kit= £179.99
- 1 x WD Black 1TB SN750 M.2 2280 NVME PCI-E Gen3 Solid State Drive (WDS100T3X0C)= £129.95
- 1 x Fractal Design Define 7 XL Full-Tower - Black Dark Tint Tempered Glass= £199.99
- 1 x be quiet! Straight Power 11 650W 80 Plus Gold Modular Power Supply= £114.95