What would happen in the highly unlikely event the Queen of the UK
were to convert to Catholocism?

Her marriage to Phil would be annulled and she would marry the Pope.

As they both have German roots, they would probably live in Berlin.

Her corgis would be sent to Battersea Dogs Home and she would get a
pack of German shepherds.

She would sell her Roller and get a Merc.

All the churches and cathedrals in the UK would revert to the Catholic
faith and her son, Charlie, would be disinherited as a heretic.

German would become the first language for UK citizens which would
really screw up all call centre staff who are still struggling with
English.
 
It would also be a mega media event that would run and run.

I can't wait!

Catholocism ... is that like Alcoholism?

Parliament would have to change the Act of Supremacy of 1559 such that
the Queen could maintain her position without having to be the Supreme
Governor of the Church.

With the changing demographics of England I don't doubt that in the
future you will see a separation of the Government and the Church of
England as the officially established Christian Church of England.

Gut instinct is that legally nothing would happen, religion affects
succession, not the continued authority of the monarch. Anyone else
got an opinion?

Extreemist bigots would murder her just like her daughter-in-law was