Migrants’ Rights are a small organisation doing big things for migrants in the UK. And they are also instrumental in the campaign about the Family Migration Rules.
A good starting point is Chai Patel’s blog on the Children’s Commissioner’s report about the Rules, which came out in September.
I’d recommend having a good look around their website, and also signing up for the weekly newsletter, which includes listings of interesting events around the UK. These include the various protests about the Family Migration Rules.