From a new pilot's perspective:
I haven't been to a comp yet, but it strikes me that it would be easier for international competitors to deal purely with WGS84 and decimal degrees etc.. rather than muck about switching between formats.
I don't see that it would be difficult for comp organisers to convert their turnpoints from OSGB to WGS84 for presentation as the comp pilots' turnpoints even if they deal with the turnpoints themselves as OSGB format.
Is OSGB used for posterity?

The use of OS grid is purely because the goals and turnpoints are picked off maps that are in OS grid.
As yet there is not enough detail on the maps in the GPSs on the hill to determine if the goal area has safe landings etc.

In years gone by the panel boughrt a lot of OS maps that are still carried about in a big box. When we landed we used to come back and stick pins in them and then measure who got where with a piece of string.

If you pick positions off OS maps, or report in a position based on an OS map, or using an OS map to find someone using their "Landranger" ordnance survey reference; need to be aware that these all use the OSGB36 datum.

I have seen on the very basic Etrex (not suitable for competition admittedly) that you could set it to "OSGB Grid" but have any datum you liked. Result - (cross country) pilots ringing in an incorrect "TM135635" style location.

In summary:

1. If using GPS in "ordnance survey" mode then needs to be OSGB Grid and OSGB(36) datum.
2. If working in long-lat, use WGS84.

First year of competition for me this year, so will be interesting to see how this works in real tasks.