The best cooler you can get under £30 is the Hyper 212 EVO, it is a legendary air cooler that has stood the test of time and is still considered the best bang for the buck CPU cooler you can get. One of the very few budget air coolers that can allow you to achieve decent overclocks on overclock friendly CPUs from both Intel and AMD.
Go for it you will love it. Watch couple of videos on how to install it on your motherboard. And my personal advice, no matter how big your case is, still get the motherboard out of the case and then fit the cooler on it. Do not try to fit it with motherboard fixed inside the case, it is difficult for even experienced pc builders.
Here is a link on how to apply Thermal paste on Hyper 212 or any other Direct Contact Heatpipe cooler, read this before you go ahead to install the cooler.
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