I they were going to wait, why didn't they call it 'First Great Western Polo on the Beach'
Probably because at this stage - lacking any complete trains in shiny new livery to show off - they want to start getting people used to the idea of the rebrand, what the marketing types call a 'soft launch', so they will do things like issue that artist's impression of a green AT300, sponsor this polo event and perhaps some other things over the summer - and bring in the new-look interiors in first class, which has been going on for months already - then, when they have the shiny trains ready and the regular passengers are all back in September, they go for the 'hard launch' with a big fanfare, reliveried trains at Paddington for a publicity and press event, new publicity material, printed timetables, etc, etc.
Various of the current timetables expire on September 6, so they have to produce new ones for the autumn anyway, so the first week of September looks like a good bet for the GWR brand to emerge.