dont know the clear fiqhi ruling wrt visiting churches purely for the sake of visiting them for looking at design etc but i know a VERY emiment world Mufti who visited St Pauls Catherderal in London just to visit it ....... though let me reconfirm that as it was quite some time since i herd that. An individual scholars actions doesnt asssume thats a thumbs up from a fiqhi perspective but again just to mention it.