A Daimler DS420 limousine has a very similar internally spacious layout to a black cab i.e glass division, occasional folding seats and offers considerably more luxory and granduer in my opinion, It is physically larger than a black cab probably internally and externally and they seat up to 7 passengers plus driver.
A Black cab will no doubt work out cheaper but you may miss out on a few things like a colour choice of ribbons and bows, uniformed chauffeur, trial journeys of your transport arrangements, floral arrangements in the rear and possibly a champagne toast for after the ceremony.
At the risk of being accused again of touting my wedding cars on here, there is a link to my company kindly posted above by another user.