I’d also recommend following Twitter accounts such as @GraphCrimes, @CoreMathsCat, @CoreMathsTom, and @AMSPLon_SE, or joining in with #CoreMathsChat on a Monday evening. I find so many topical resources on Twitter, and I’d also recommend simply searching #CoreMaths.
Finally, and this isn’t a resource as such, but Mathematics in School (published by the Mathematical Association) has just released its Golden Anniversary Issue. This issue takes a slightly different perspective of ‘Mathematics OUTSIDE School’ and looks at some of the benefits of Core Maths. It’s definitely worth a read!