// Where will this 'takeover' end //
I agree, our High Streets have far too many American eateries.
// The sandwich chain said 'following a strong demand from our Muslim customers', 185 outlets in the UK and Ireland have introduced the meat, which is prepared under strict Islamic rules. //
Subway are responding to their customers preferences. They could ignore their customers, but will lose money. As the purpose of these restaurants is to make as much money as possible, then they will supply food that their customers want.
If there is a backlash and people start the boycott the horrible places, then they will start to lose money and they will change there minds.
Market forces at work. There are a couple of Subway branches near me and they do seem to attract a lot of Asian customers, possibly they have using halal products for a long while