Afternoon Nag,
You could look at the Slough Grammar school consortium:
https://www.sloughconsortium.org.uk/ .
Although we are waiting for the link for this year's results (with chewed finger nails in my case lol), once we have had it then they will show what you need to do for entry in sept 2019.
My dd already attends St Bernards Grammar, she has a child's photo oyster card and catches the number 81 from the bus station.
The card cost me about 15 pounds and I know I had to put some money on there as well in case she catches the tube at some point. Otherwise the bus is practically free. The pass is an 11-15 year old, so will last her quite a long time.
It is a popular bus with both the Upton and St Bernard children. Your dd would still have to catch the train from West Drayton so you would have to see how frequent they are.
Or check the school links through the above website for their transport policies as well as admissions/ catchment criteria.
Good luck.