Subsitutions for Monterey Jack cheese in the UK
Monterey Jack is in no way cheddar.  The advice to use Gouda was about as good as you can get. (make sure it is neither smoked nor aged).  Sainsbury has a four cheese pre-shred mix with MJ, Aged Cheddar, Edam and mozz that will approximate those four cheese mixes found in most US grocery stores and the finer 7-Elevens but MJ itself is near impossible find.