I live in Watford WD postal code my DD is taking her exams next year is she allowed to take Berkshire Consortium exams.
You are allowed to take exams in any consortium anywhere in the country. In a different forum in this website, I read that one child from Yorkshire appeared for exam in Bucks
Most probably you wanted to know whether there is any chance of your DD getting a place in Berkshire GSs if she takes the consortium exam.
This has already been addressed by Reading2015.
You need to be in the catchment for Kendrick (by the closing date of application form , which was 20th June for 2015 entry, you have to find out what that will be for 2016 entry though). WD postcode does not come under that unless you decide to move into Kendrick's catchment. Langley has a catchment, please check it's website whether your postcode is included in that or not. If your postcode in included then there is no problem.
For both Herschel and Upton Court there are about 70-100 seats at each school which are open to OOC candidates based on ranking only. Therefore, you can always take a chance for those places.
Hope that helps.