no it is not, they do not check for a catalytic converter on the car during the test - look at the checksheet
all they check is the exhaust emissions, if you look through any information booklet, not once does it state that a cat must be fitted to a post-1993 vehicle
all it will say is that the emissions are within a set boundary
do some research on this, and see what the results are
and FYI my saxo vtr passed its 1st MOT (yr2001) with a decat fitted - was done at a citroen garage, im sure they would notice if there was no cat