Berkshire does not have a 12+ per se.
For the schools that you mention you approach the schools and your child will sit an exam set by the school. Thishas previously been curriculum based I believe, but the schools should inform you of the basis for the exam.
Langley / Herschel has a section in their 'admissions' that does not allow any child to take an entrance exam for entrance in years higher than Y7 if they had already taken the 11+ for the consortium.
Pupils applying for â€˜in year' entry between Year 7 and Year 11 to the Consortium of grammar schools, will be subject to tests set by the individual grammar school and they must not have sat the Consortium 11+ tests.
Slough Grammar does not have this section in their admissions criteria.
Admissions after the initial year of entry will depend
upon the availability of a place and suitability of the
pupil for a grammar school education as determined
by admission tests. This includes admission to the
Sixth Form where the relevant admission test will be
performance in GCSE examinations or NARIC verified
Hope that helps