Hi Snife,
As I recall it, Liverpool's tall buildings policy was created as a means of protecting the World Heritage Site and it's buffer zone by restricting tall buildings to certain specific areas.
I don't know how a tall building was defined (if indeed it was) but these buildings were to be confined to three areas of the city centre. One of these was the Old Hall Street / King Edward Street / Leeds Street area (north of Chapel St / Tithebarn Street), an area that already had several tall buildings and which has added six buildings over 15 storeys this decade.
The second site was the area around St Johns Beacon, which included the now abandoned Lime Street Gateway tower and the Central Village towers, which are planned to be on site this year.
The third site was the southern part of the Kings Waterfront development, where three residential towers were proposed (although the scheme is now being reconsidered).
It became immediately apparent that the areas were not properly defined and seemed to refer to specific projects only - ones already approved such as the Lime Street Gateway scheme. In fact, a tower proposed for an adjacent site by a private developer was turned down simply because of its proximity to the official proposal.
I suppose that the problems with the proposal stemmed from the fact that it was never clear whether the zones marked areas where tall buildings would be encouraged or just tolerated. There was also the question of what happened to tall building proposals that fell outside these areas - as many did. For example the Cesar Pelli residential tower that was lopped from 22 storeys to 17 storeys on English Heritage's insistence. There is also the case of the Brunswick Quay tower that was well away from the WHS or buffer zone but was cancelled after a prolonged fight that included a successful public inquiry whose recommendations were rejected by a government minister who hadn't even visited the site.
Even within the tall-building zones, buildings were still subject to arbitrary storey lopping and the tall buildings policy has generally been seen as a means of preventing the construction of tall buildings in Liverpool than a positive attitude toward high-rise development.