"Unholier than thou"

Friends are getting married in a Registry Office, but have asked for a Christian service of blessing as part of the ceremony. The law forbids any religious input into a secular ceremony, so I'm having to devise a service that starts with the 'legal bits' done "godlessly" by the Registrar, who then leaves, and then continues with promises, exchange of rings, blessing etc. But we didn't want the legal promises to be completely sparse. So we suggested that the following was said :

"I XX take you XX to be my lawful wedded X (i.e. the legally required words)
to have and to hold
from this day forward
for better for worse
for richer for poorer
in sickness and in health
till death parts us"

No reference to God, and nothing religious in that, but the Registrar deems it unacceptable. The reason? It's a quotation from the Book of Common Prayer (which it sort of is, albeit with the line "in accordance with God's holy will" removed. Actually these lines are found in the wedding services of most Christian churches in the UK, I'd imagine.)