Ted Given that most customers will want prints, then quite apart from the lack of exposure latitude with slides, I'd have thought that the price of Cibachrome prints and lack of printing sources in the U.K. might be the main reason that colour neg is used.
PE, I think, would go further and contend that nowadays colour neg and RA4 prints will give better prints than Cibachrome anyway.
It might be that if photographers can ensure that exposures are spot -on then slides can be printed onto RA4 in mini-labs but even then the slides printed onto RA4 would have to be better than colour negs for the risk to be worth it.
Unless the quality difference with slides printed onto Cibachrome or RA4 is appreciably better than colour neg on RA4 then the reason for wedding photogs not taking slides will be simply: Why take the extra risk with no measurable pay-off.
In the wedding business there's no second chance as in: Sorry all the pics weren't as they should be but never mind we can do it all again next weekend :