I once had to be helped with lifting my bike out of the mud on Combe Gibbet in Winter! All of the ridge should be avoided. As regards Watership Down (one of my favourite areas), bear in mind that MX’ers use the section between the A343 Ashmansworth and the A34. The bit between the beech wood and the A34 can be lethal. Past the underpass will be fine up to White Hill.
May I suggest the area South of Combe Gibbet? It holds up really well in Winter, ie Vernham Dean. Loads of rights of way in the area. Check out this link:
http://www.testvalley.gov.uk/Default.aspx?page=9586
If you look at the ones starting from Vernham Dean, you should be able to incorporate these in your mega-ride. I know this area really well so if you need any more info, e-mail in profile. 🙂