things may have changed, but the NHS would not let local authorities email NHS Trusts anything with patient data in it as this was only permitted within the NHS using their own secure email system
Amusingly enough, the NHS also wouldn't let the NHS email patient data. Because the central nhs.net service was encrypted but local Trusts' nhs.uk setups weren't necessarily.
Things have changed, a bit. See: https://s3-eu-west-1.amazonaws.com/comms-mat/Training-Materials/Guidance/encryptionguide.pdf (PDF, soz)
Seems like a bit of a faff to set up to be honest, so I wonder how much it gets used. But it also looks like they were aware of the problem and tried to address it.