Hi, are you looking for a full boarding, weekly boarding or flex-boarding school? All boys or co-ed?
Unfortunately, I don't know of many boarding preps in the south west, if you're willing to go further out you might find a wider selection. I also don't know of any that feed to Winchester College in that area.
Here are the few preps I know of:
Mount House School: Co-ed. Website here. For boys, seems proud to feed to Eton, but also mentions some boys go to other schools including Winchester (see here). Doesn't seem to "macho" or "jock culture" to me, but I have no experience with the school.
Tockington Manor: Co-ed. Website here. Feeds to a range of schools. It doesn't mention Winchester, but does talk about Eton and Harrow, so maybe not too much of a stretch?
Sherbourne Prep: Co-ed. Website here. Doesn't really mention what schools children go onto, or at least I couldn't find it. Good school though, from what I can see.
Finally, Wycliffe. Not a prep, but has a prep division. I don't know if you would consider it because it has an attached senior school but the website is here for you to look at.
If you want a single sex, (almost) full boarding Winchester College feeder prep I think you'll have to look to the schools outside of the south west such as Ludgrove, Sunningdale, Horris Hill etc.