Old thread but I can give some insight as we have a DD who started at Brighton College last summer and weekly boards.
About thirty girls board per year (from a total of 80ish girls in the year) - dont know the precise figures. Girls seem to be about 40-45% of the total (my guess); presume there are other attractive girls schools around (seem to be quite a few on the hockey / netball circuit).
As you can probably tell, we don't know the area at all well as we are one of the London contingent (sizeable number) who have gone down the weekly boarding route. Other boarders are from the Sussex region and a small (1-2) number of Chinese etc (who fully board).
Our DD travels independently from central London (often meeting friends on the train either on a Monday am or a Sunday pm). Sport matches on a Saturday can involve staying over on the Friday or travelling down on the Saturday.
Boarding is well-organised with a rotation system in the choice of room-mates, and the house system works well (big fan of younger girls mixing with older ones in joint ventures etc).
DIscipline is based on mutual respect but there have been a surprising number of expulsions in older years (not something we have experienced in London day-schools).
Overall, our experience is good. The pupils seem well-balanced, polite and 'ordinary' (within a mono-cultural home counties population). The Headmaster is smooth, proactive, ambitious and fastidiously on-message. Quality of teaching seems to be good and the prep workload not especially onerous as compared to a couple of academic London day schools I could mention.
Communication and marketing is much superior to other schools we have gone through with weekly news letters running to 10-15 pages and the IT is impressive. Sport at the higher levels is impressive and can be nationally competitive. Music and drama are, I think, also satisfactory but TBH i have really got a feel for them yet.
One issue I would raise is that teacher recruitment may be problematic. If a gap arises mid-term, significant delays in identifying a replacement has arisen (? geographical limitations). Provisional GCSE choices were also requested remarkably early to allow planning for teacher numbers.
Academically, the standard is high and improving and notably the school is attracting better pupils (apparently this year has the highest CAT scores / MIDYIS).
Perhaps my most favoured impression is the sheer breadth of activities and events for the children. The opportunities are immense and day-pupils often stay well into the evenings from what I can gather.
Hope that helps