this website might help. I've only had a quick look but it lists a lot of schools and gives info on their numbers etc. Personally, if I was looking for a boarding school for prep aged children I would look at activities done outside of the school day. I think that they should be kept busy with more structured activities than at a senior school age.
I would also want to know proportions of day:boarding pupils, in part because I think that will affect the emphasis given by the school on my first paragraph.
I would ask about the pastoral care structure. Are the children in houses? What ages are kept together? Do they mix with children of different ages? How many children are in a 'unit'? (whether it be a house system or year group structure) Who looks after them and what are the processes in place for ensuring their pastoral care? How is this linked with their academic care?
I would want to know if most children go home at weekends, or what are exeat arrangements. What is arranged for the children to do at weekends? Do the children get to mix informally with resident staff and their families?
I would want to see if the boarding facilities felt like the children had ownership of them. Could it feel like your child's bedroom? Would they be comfortable and relaxed there? Could they put their own mark on their space? Do they have anywhere they can call their own? What about if they just want some privacy and quiet? Is that possible too?
My dc are day pupils at a boarding school. There are some children boarding in their department but most do not start boarding until Year 7 or 9 (on entry to the senior school). However the school structure is set up for the benefit of the boarders (as it should be imo) so all pupils are required to be in school from 8am to 4-5pm 6 days a week. This ensures that the boarders do not have hours and hours of blank time each day where they are at a loose end. My dc never want to come home at the end of the day either - I have to drag them away from their houses because they would prefer to play table tennis, use the computers, have a quick game of football, go to school supper etc etc. I think the boarders feel very at home and relaxed.
I think if I were looking for a prep boarding school for my dc to board in, I would look for a stand alone prep school, rather than a prep dept of a bigger school, as I would hope that they would be more geared up to the specific needs of younger children in a boarding environment. Just my opinions of course, and have no particular recommendations, but hope it helps a bit.