I'd read the OP as someone currently in TW looking to possibly move to Crowborough once their child had a place at a grammar.
I'm sure it's possible for travel from Crowborough to TW grammars - there are many Bennett and St Greg's children who live in Crowborough and travel in each day and also boys who travel to Skinners'.
I haven't check but I thought it allocated places on distance. I'm not sure if it gives priority to KCC residents.
Boys Grammar will only take children from out of county once all Kent boys in a selective area have a place. Then they will take OOA/OOC on distance. Occasionally, like this year, they will take children from Sussex but it is becoming a rarer and rarer thing.