The Monitor Audio is an All Weather (outdoor) speaker. You don't really need that just for a bathroom. It's never going to get down to freezing temperatures, you hope
If the BCK65-SS will fit then that's the one to buy. It's a cracking speaker. Everyone that has bought them from me have been delighted.
Re: your install. If the current ceiling is intact then that should act as a firebreak for any living space above. While you have the chance, have a chat with your builder about fitting a frame in ceiling to help with the acoustic mass loading of the speaker.
It's nothing too fancy or complicated. Just a rectangle of MDF or rough sawn timber; 100mm high, and with an internal dimension of say 477x313. You're aiming for something with an internal volume of 14~15 litres, so the length and width dimensions can be altered to suit. Just avoid anything where one dimension is an even multiple even multiple of another. e.g, the height is 10cm, so you wouldn't have a length or width equally divisible by 10. Same goes for the length and width. 477/313 gives us a horribly complicated number - 1.523962 - and that's good for acoustics!
Put the frame up on the ceiling before the boarding happens. Seal the upper edges with intumescent sealant. Drill a hole for the two speaker wires. Pull a good metre to metre and a half through, then seal the hole with the sealant too and coil the speaker wire ready for the fit.
Once the boarding is finished you (or they) will cut the fitting hole - 210mm diameter - and at that point the lower edge of the frame can be sealed against the new boarding. Chuck in a couple of bits of insulation - doesn't need to be filled to the brim, just stuff the far ends to help break up any sound waves internally. Then fit the speaker. You'll have tigher bass than if the speaker were installed in the open void, plus it stops spiders etc crawling over the back of the speaker.