You state a budget of £340. When it comes to upgrades, as far as I'm concerned, it depends on what sort of performance increase you're looking for and how you want to upgrade.
Performance-wise with what you're had suggested, I'm not overly convinced the switch to the 2400G is worth the cost. But you will be on a newer platform and be more future proof. If you're unlucky you may have some issues with the 2400G due to BIOS compatibility, though AMD do have a service to help you flash the BIOS if required (I assume we have that in the UK as well).
The Coffee Lake alternative, at the time of writing, still seems to lack the cheaper motherboards. This could be an Intel option, but bear in mind that the thought is 4 cores are becoming an issue for some games:
https://uk.pcpartpicker.com/list/4fLw6s
Consists of an i3-8100, z370 motherboard and 16GB of RAM.
Both of these are more future proof due to being on their latest respective platforms. The alternative would be to upgrade your i5-3330 to something better on the same motherboard.
As with any build, if you have the right basics you could upgrade certain components. Which is more viable will depend on what graphics card you're using now and intend to use again. It will also depend what sort of games you play. A lot of variables to consider, but these are just general considerations on my part.