From AMSP “What is Core Maths?” provides a really useful summary of this level 3 qualification which is equivalent to an AS level qualification including links to the qualifications available. Also from AMSP see Core Maths curriculum which includes their Core Maths fact sheet and a very useful summary of the content of the available qualifications. See also a useful set of links to resources for Core Mathematics.
I have found many of the STEM Learning resources useful for higher GCSE, see this post on Averages and Spread.
On this page of Decision Mathematics Teaching Resources I included core maths resources as a useful source of materials. Looking at AQA’s Certificate Level 3 Mathematical Studies, we see from the specification that one of the optional papers is on Critical Path and Risk Analysis and I see Linear Programming on Pearson Edexcel’s Mathematics in Context (Level 3 Core Maths) qualification. See for example Edexcel’s practice questions. On STEM Learning check the Nuffield Mathematics resources where resources are recommended for each qualification.
On Jack Brown’s TL Maths you will find resources for Core Maths, this very useful page on resources includes links to many resources for Core Maths and Jack’s teaching videos include a Basics section suitable for all specifications and two other sections are focused on AQA Mathematical Studies.
On Twitter there is a Core Maths chat fortnightly in term time. Follow Catherine van Saarloos, @CoreMathsCat, and Tom Rainbow @CoreMathsTom. Note the Summer Core Maths Festival and a great opportunity to join a network of Core Maths teachers.