Are there any hard and fast rules in the building regs that state, for a
standard 2 storey domestic dwelling...

1) How big an external rear door (that you might need to get out of in the
event of a fire) needs to be, e.g. 78" x 30" min or whatever?

2) Whether a 2nd WC (1st WC being inside in a bathroom) can be a downstairs
outside loo (ie no internal access).

Or would any issues along these lines be down to the opinion of a building
inspector?

Paul.