Hi,
Sorry I'm a bit slow, what does DTS stand for?
In regards to prep boarding, I assume you want full boarding? Is there any particular area you want a school in?
Of the schools you have listed, this is what I know:
Moffats Shropshire: very small family run prep school. About 20 boarders. Good atmosphere, happy children but VERY small.
Malvern St James: good independent school, goes right through to the end of sixth form so good if you're looking for stability, allows sisters to stay together for a few years. Girls only.
Queen Ethelburghas: Coed, quite big, goes through to end of sixth form. Very good on paper but I have heard mixed things about it. I don't have any first hand experience with it though.
Royal Alexandra Reigate: Large and quite popular. It is a STATE school which is unusual for boarding and may be a benefit or a drawback for you. You don't pay tuition (though you do pay boarding) so it is cheaper than the independents. Again, co-ed and goes right through to 18 years old.
In regards to prep schools, I would recommend you also look at:
Hanford Prep (Click here) I have never heard a bad thing about this school. All girls, only goes through to the end of year 8. Independent.
Windlesham House School (Click here) Very good reputation. Co-ed, 3-18. About 300 children but I don't know how many are boarding.
Other schools I don't know much about but have head positive things are:
Godstowe and Cheam.
Good luck for what you're looking for.