I feel if you put Langley as first choice and if your child does not get through in Langley then there is no question of admission in any school in Berks county.
If you have put BG as next preference to schools from berkshire automatically this becomes the first choice in Bucks county.My feeling is he will get admission in BG.
The question is if BG gets more than 120 children then the question of marks/catchment/siblings comes into play.
If your child gets through Berks test then the child becomes eligible to get admission in both Berks and Bucks.Since the first choice is Berks naturally you should get there. The LEA you belong to will discuss with both the counties and decide, here iam assuming you belong to third county.....ha ha