Considering your current specs, I said start with CPu+Motherboard+RAM, and then save up and upgrade the GPU.
Also
DON'T buy a Core i5 9400F, not now!!! Intel is launching new CPU later this month (lets hope), they will launch new Core i5 with 6 cores and 12 threads, when the 9400F only have 6 cores and 6 threads. Besides if you wana buy that one it may drop its price aftter the new ones comes to market. (btw intel killed that 9th gen platform, for the new cpus you need a new motherboard, so youre basically buying a dead platform in every point of view).
If you really wana go with intel, and you wana do it now, wait till they launch thier new models.
Now if you wana get good performance for good price AMD is always an alternative, I will leave some systems for you to compare:
Your current idea (not advisible, only 6 cores/6 threads for a dead platform, you may need to make sure the motherboard BIOS is up to date to support the CPU you choosed):
PCPartPicker Part List
CPU: Intel Core i5-9400F 2.9 GHz 6-Core Processor (£139.98 @ Aria PC)
Motherboard: Gigabyte B360 HD3 ATX LGA1151 Motherboard (£92.59 @ Overclockers.co.uk)
Memory: Corsair Vengeance LPX 16 GB (2 x 8 GB) DDR4-3000 Memory (£72.95 @ Amazon UK)
Total: £305.52
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2020-05-03 18:43 BST+0100
My advice 1 (lowest budget, still way better future wise than any budget current Core i5 build):
PCPartPicker Part List
CPU: AMD Ryzen 5 2600 3.4 GHz 6-Core Processor (£114.14 @ Senetic)
Motherboard: MSI B450M PRO-VDH MAX Micro ATX AM4 Motherboard (£73.48 @ Amazon UK)
Memory: Team T-FORCE VULCAN Z 16 GB (2 x 8 GB) DDR4-3200 Memory (£76.97 @ Laptops Direct)
Total: £264.59
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2020-05-03 18:39 BST+0100
My advice 2 (best current mid range CPU in the world, atleast for now, according to all the reviewers):
PCPartPicker Part List
CPU: AMD Ryzen 5 3600 3.6 GHz 6-Core Processor (£158.98 @ Aria PC)
Motherboard: MSI B450M PRO-VDH MAX Micro ATX AM4 Motherboard (£73.48 @ Amazon UK)
Memory: Team T-FORCE VULCAN Z 16 GB (2 x 8 GB) DDR4-3200 Memory (£76.97 @ Laptops Direct)
Total: £309.43
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2020-05-03 18:39 BST+0100
Keep in mind that AMD is to launch new CPUs in september too, and it seems most B450 chipset motherboards will support them too (after a BIOS update), so you will have a robust upgrade path there with the Ryzen 4xxx later on.
Cheers!