I thought it was still against the law for catholics to be PM. Elizabeth I passed that law, but things seemed to have changed over the years.
hXXp://wiki.answers.com/Q/Can_you_have_a_Roman_Catholic_Prime_Minister_of_Great_Britain
There is no legal mechanism in place for a non protestant to be PM, which would cause a "constitutional" crisis situation.
hXXp://news.bbc.co.uk/1/hi/uk_politics/3746398.stm
Even Tony Blair thought that catholics can't be PM of the UK.
Disraeli was born a jew, but converted to CoE at the age of 12.
The monarch can not be or marry a catholic.