It's a criminal offence not to provide complete and accurate information on an electoral registration form. The maximum penalty is a fine of �1000.
Credit reference agencies are obliged to ensure that they do not create an 'association' simply because of a common address. (e.g. If Fred Bloggs moves into a house formerly occupied by Joe Smith, who had bad debts, Fred mustn't be blacklisted for credit just because of his address). However, they are permitted to create an association where there is reason to link the finances of two people at the same address. (Marriage is an obvious reason). So it's extremely likely that your wife would be refused credit if she applied for it.
However, the first thing that credit reference agencies do when assessing people for creditworthiness is to check that their name appears on the electoral register. If your wife applied for credit (and you'd failed to include her name on the electoral register), the agency would see that her name was not listed at the address she gave on the application form. So she'd almost certainly be refused credit anyway.
Chris