In theory it is based on the unimproved value of the land with whatever zoning laws are in place on that land.
So if you owned an empty plot that had planning permission to build a 10 story apartment block then that would be worth more than an empty plot that could only have a single dwelling and your tax would be higher, this is a positive thing because it discourages developers from land banking and doing nothing.
If you had a normal house with garden, then the taxable value would be what that sized plot on that street would be theoretically worth if it had no house on it (given existing zoning laws and any planning permissions). This is good because it means you can improve your property and extend your property and your tax doesn’t go up. You’re not punished for investing in your asset.
It would for sure be more complex to manage and more open to disputes when the value is inevitably set at the wrong level for some properties. But we have so much data available now, so I don’t think it’s an impossible task.
The one benefit of stamp duty is that it’s very clear cut, you’re paying a percentage of a transaction. But just because it’s easy doesn’t mean we should indefinitely ignore all of its harms.
Probably what will happen is they will instead introduce a lazy property value tax (rather than an unimproved land value tax) so people will be taxed more the more they improve their property, which will suck, but at least slightly less than stamp duty.
I have absolutely no faith in any UK govt doing something sensible and well thought out!