Not one of these had included a clause about changing locks.
And also the part about "you will get evicted immediately" is nonsense. The process to evict a tenant takes time: the landlord need to serve notice, then wait X days for a response, then apply to the court, who will then (assuming the landlord has followed the law to the letter) issue an judgement, and then the landlord still cannot have you forcibly removed until some period has passed and they have applied to the court for an enforcement order.
If you have had the experience you describe with more than one landlord in the UK, I'd suggest you look through a different agent next time.
The others are, as far as I am concerned total scammers. I won 3/3 cases in the Ombudsman, so I say this for a reason. Cheffins went as far as taking all my deposit to cover some debt the landlord had with them. And of course they didn't tell me. Simply no reply, no deposit. Even the Property Ombudsman freaked out.
I've only once rented a flat in Oxford. It was over 10 years ago, so I don't recall the details, but I do recall having to argue with the letting agent about changing some wording which was totally unreasonable.
Perhaps the situation is different in university towns, where lettings agents often dealing with naive tenants who have never signed a lease before.
The landlord can't evict you because of this, and the only reason they'd know you've changed the locks because they've tried to enter the house when you're not there.
It's pretty sad. In my home country, it's a very serious crime if your landlord breaks into your house for no reason. Actually, landlords generally do not keep a key to the property during tenancies precisely because of this.