Christening a have always been a bit of a piss up round here, too.
I don't have a problem with byob, most parties I go to our byob other than lager and boxed wine...however, unless your house is close enough to drop of first, people may feel uncomfortable bringing bottles into church.
I think I would go with:
Enough fizz for everyone to have a toast - you get about 6 glasses per bottle so I reckon 7 bottles for 45 people max, but would probably go with six.
1 bottle of wine between three, on the assumption that not everyone will be drinking, so about 15 bottles. I'd probably go five each for red, white and rose, although you might get a discount for buying by the half case.
And probably six cases of beer split four lager, one beer, one stout.
If it's summer time, I would probably do a couple of jugs of pimms and lemonade. Tescos do a rip off version called Jeeves which is about half the price
And soft drinks,mod course.
Although if most guests will be driving, you'd get away with less.