I have made an official complaint to the financial ombudsman. I suggest others do the same. IG were well within their rights to increase margins to manage "risk" although the risk is zero at 100% margin... but changing their terms and conditions and then mistreating their customers is pretty poor form. Don't take it lying down.
https://www.fca.org.uk/consumers/how-complain
https://help.financial-ombudsman.org.uk/help
Here are IGs details for the complaint:
IG (IG Index Limited and IG Markets Limited)Cannon Bridge House25 Dowgate HillLondonEC4R 2YAUNITED KINGDOM