It's unlikely to get 200-400MB/s from any CF card over USB3.0 though.
I have a good, fast CF card reader and Sandisk Extreme CF cards... along with Sandisk Extreme SD cards... the most I ever see is 120-140MB/s read speed and that's straight to a raid-0 NVMe array capable of writing sequentially at 2500MB/s
Still better than the 30-40MB/s you get from USB2.0 though.
For the OP... Like others have suggested, a replacement second hand motherboard so that you can overclock your CPU (your Gelid CPU cooler is good enough for a good overclock, you don't need to replace that - just make sure you have some new thermal paste when you swap).
SSD will be a massive upgrade... it ideally replaces your main hard drive and it is possible / easy to clone the drives so that you don't have to install everything from scratch.
With replacing the motherboard though, you will have to get a new version of Windows (unless you have a full retail version of Windows 7 Home already) as the OEM version is licensed to the motherboard. If you have the retail boxed version of Win 7 Home, you can move that to a new computer.
With only 8GB ram, you will see an improvement upgrading to 16GB... but beyond 16GB you won't see much benefit in lightroom - so don't upgrade your operating system for that alone.
For £250, I would get:
£50-100 - New/second hand motherboard to overclock your CPU
£50 - 16GB RAM
£125-130 500GB SSD (Intel, Samsung or Crucial)
Giving you a total expense of £225 to £280.
Although it is possible, as mentioned above, that you may also be forced to purchase a new version of windows with the motherboard upgrade - which might add to the cost. However, the person selling the motherboard might have a copy of Windows that is linked to that motherboard and they might be willing to share.