From a personal perspective, I would think it highly unlikely that you'd get much of a market for HO (and hence a return on your investment) for far too many years after the introduction of your first HO RTR model. The problem is this: in OO, there are several UK outline manufacturers, each of whom is producing several models a year and each of whom has been doing so for some time. Thus, there is a broad range of locomotives, coaches, wagons and now even units from which the beginner can take their pick. Moreover, all of the Big Four companies and BR are well represented (compared with HO) in OO models, allowing for the modeller/collector to have some geographical variety in their fleet. With HO, however, any new manufacturer (or existing manufacturer moving into UK HO) has to contend with this lead which OO has and which OO will continue to build on as manufacturers tool up more prototypes. All this makes OO much more attractive for the wider public, who don't really mind the fact that OO is prototypically incorrect. That we are now seeing pre-Grouping and oddball diesel prototypes appearing in OO indicates just how much of a lead OO has on any challenger. An HO range would be stifled by this competition, not least because there's not even a wide range of trade support in terms of kits to back up the RTR produce and because it would take a while to build up a useful range of prototypes which can be used in a layout of a particular era or region.