This isn't always true, though it's typically only not true when operators are throwing ridiculous offers at people who are trying to leave them. Similarly, I got phone a few years back at a ridiculous student-only deal. Both cases ended up with me paying about £200 more over the whole contract than I would've with SIM-only contracts, and both got me phones that retail for more than double that.
For any bundle they are offering they should have a non-zero cost in acquiring the phone, in which case they'd have a very hard time to justify not letting you sign up to an unbundled agreement where at the very least the cost of the phone is removed. They'd almost certainly be breaking the law if they do (of course that doesn't necessary mean they aren't doing it though)
This isn't always true, though it's typically only not true when operators are throwing ridiculous offers at people who are trying to leave them. Similarly, I got phone a few years back at a ridiculous student-only deal. Both cases ended up with me paying about £200 more over the whole contract than I would've with SIM-only contracts, and both got me phones that retail for more than double that.