geofelt :
Your A6-7400K has a passmark rating of 2795 and a single thread rating of 1567.
The proposed X4-860K is the strongest cpu supported by that motherboard.
It has a passmark rating of 5633 but only a 1613 single thread rating.
If your main use is for batch applications, the extra cores will serve you well.
If your use is for gaming, it is a somewhat sideways move because few games can use all 4 threads.
Regarding for gaming, AAA games after 2012 (after Tomb Raider) are mostly uses 4 threads. So unless you play below AAA games, then I have to disagree. Even some current AAA game has started using 6 and 8 thread as their base, making them well known as i5 killer. Don't get me wrong, i5 still work on those game, but at certain point in game, it stutter a bit. Game that I know as i5 killer are Battlefield 1, Farcry primal, Mafia 3, and watch dogs 2.
And again, back to OP, your motherboard maximum upgrade is either X4-880K (not 860K) or AMD A10-7890K. Source link as follow:
https://www.asus.com/id/Motherboards/A68HMK/HelpDesk_CPU/
Here are the link of 4 cores FM2+ in your country
https://uk.pcpartpicker.com/products/cpu/#X=0,10827&m=6&k=26&sort=a7&page=1&C=4,22
with 100 poundstrling limit, A10 is out of the picture (also not economical)
Now as realistically upgrade for this motherboard, I would suggest you just get either
65 poundsterling AMD Athlon X4 860K 3.7GHz
https://uk.pcpartpicker.com/product/kWGj4D/amd-cpu-ad860kxbjabox
or
80 poundsterling AMD Athlon X4 880K 4.0GHz
https://uk.pcpartpicker.com/product/94s8TW/amd-cpu-ad880kxbjcsbx
For both cpu, you can just use their standard heatsink. No need to buy additional heatsink. I agree they are suck but its really not worth the money.
If you still concern the heatsink, then you will probably go directly to X4-880K as its standard heatsink is better version.
https://www.youtube.com/watch?v=fWic723XBxk
It came with 2 pipe copper heat sink with all copper plated base. Although unfortunately its still horisontally build, but I can bet its far better than those standard heatsink in X4-860K that has no copper at all (I have this one for my X4-B55 Phenom 2, and its really sucks)
https://www.youtube.com/watch?v=tcFH7g_YpXQ
with 15 poundsterling different, you'll get better clocked base cpu and better heatsink.
Then again, its still an outdated cpu, but if this is all you have, then make the most of it. Its 4 core should boost up your overall performance in cpu wise. Please do care about the power supply when you applying cpu on those upgrade.
Good luck with your choice