Unfortunately the Coombe Hill race hasn't been on for the past couple of years, since the RO decided to call it a day. Shame, as it was great little race!
As far as other Chilterns off-road races with hills are concerned, Watlington Hill-bashers do a very low key trail 10k both summer and winter, which is probably the closest thing to a fell-race in the Chilterns, both in terms of climbs and ethos. There are also a fair few other trail races that have plenty of climbing, but certainly no navigation element, such as the Ridge Off-roader, Run the Rock, AShardeloes 10k, and Rugged Radnage. Funnily enough these are all 10k and most are fund-raisers for schools, but still tough little races of their ilk.

As far as navigation is concerned, weekends away oop north or to Wales, ideally with more experienced mates, is one way or how about doing one of the FRA navigation weekends?