Hi Jack.
Smart Meters are a tricky subject that I experienced with my last tenants.
After moving out and I took meter readings to my absolute surprise I found both my meters had been changed to smart ones.
My tenants nor estate agent ever advised me this happened nor actually asked my permission to change them.
When I queried this with the agent I was told as they were the tenants using and paying the utilties at the property they could basically do whatever they wanted (despite not being responsible for the structure of the building). I couldn't believe this and after googling I found Ofgem advised if the Landlord pays the bills it's their decision, if it's the tenant's paying a Landlord cannot unreasonably prevent it.
https://www.ofgem.gov.uk/information-consumers/energy-advice-households/getting-smart-meterHowever it seems that with my potential new tenants moving in, I put in the addendum they shouldn't change the meters without permission, the estate agent turned around and said this was in the tenancy agreement already! Which if this had been the case with the first tenants I had, they wouldn't have been able to change it eventhough they just did what they wanted without asking me first!
So it seems I wasn't able to get a straight answer from my agent. But I'd advise to check your tenancy agreement, if it states they're not allowed to change them without permission and they have signed this agreement. They've agreed to this clause.