This doesn't really require a smart lock. In Germany it's common to replace the lock immediately once you rent a place and put the original lock back in when you move out. This prevents the landlord, previous tenants and whoever else might still have keys from entering without your permission.
One can easily pay £500 ($700 USD) to change the locks on a home in the UK.
This guide has an illustration: https://www.obi.de/magazin/bauen/tuerschloss-wechseln (text is German but the pictures show everything)
Perhaps in the UK different kinds of locks that are harder to change are more common?
When I replaced my locks at a previous apartment I paid $40 for the new lock and installed it with a single screwdriver in 15 minutes. Multiply that by 2 if you have a house with two exterior doors. Multiply again by 2 if you want nicer locks.
The majority of residential homes in the US have semi-standardized lock bore and setback sizes so most anything you pick up for dirt cheap at your local home improvement retailer will bolt right on. I'm not sure if there's the same level of standardization in the UK, but in most of Europe locks are of a design that allows you to replace the cylinder with a couple of screws from the inside. The locks themselves typically cost more but the labor is pretty much nonexistent.
The only way you'd need to pay that much to have locks changed is if you're needing to do actual physical work on the door or jamb itself.
Or if your locksmith is overcharging you because you don't know any better.
As a tenant (and a hotel guest) I've used both of these methods over the years. Granted, I've never lived in a home that has a smart lock, but I know I would feel much safer using my own hardware for the purpose.
[0] https://play.google.com/store/apps/details?id=org.havenapp.m...
This has a massive potential for abuse. Imagine if your landlord could just take away access to your appartment remotely whenever they want.