He wouldn't be an heir under the current rules, apparently. Only those who at the time of succession are Protestants (ie CofE) can succeed to the throne -- so he would have to convert before succeeding.
I don't think that would work. The Monarch is head of the Church of England (and Scotland ?), so I don't think they'd be a fan of an atheist at the head of their Church -- and what atheist would want that role anyway?
I was thinking of a not-dissimilar question the other day.
What would happen if a republican were elected PM? It's unlikely given that public opinion is broadly pro-monarchy, but just hypothetically, what would happen if the PM refused to go to Buckingham Palace and ask to form a government? Would he be required to step down? Would nothing happen?
presumably any republican elected to be PM would have campaigned on a platform of abolition. Thus at the time of coming to power he/she would begin to enact that policy. also at that time the monarch would still be constitutional head of state, so the PM-elect would still have to go to the palace - even if the opening gambit were to be "Hi Maj - you're history".