Does this mean that out of County chilfredn will get preference over Bucks county children?
I think the answer to your question is that out-county children in the catchment area should get preference over in-county children who are not in the catchment area.
In the light of the Greenwich and Rotherham judgements, BCC cannot discriminate specifically in favour of in-county children, but
they can lawfully give priority to in-catchment children (and, as you have pointed out, there are instances where the catchment area crosses over local authority borders).
Catchment areas are reviewed regularly, and one would expect a consultation period before any changes are implemented. All sorts of factors are no doubt taken into account, from historic patterns to demographic trends.
PS: You might like to look at the thread "Out of area allocations" 29/3/06.