Thof, I am coming late to this thread but I can confirm / add a few things.

There is some hilly running in the north eastern part of the Chilterns not far from you - e.g. Dunstable Downs, Ivinghoe Beacon, then slightly further afield at Wendover Woods and from Coombe Hill down to Whiteleaf Hill. I can give you some suggestions for routes - PM me.

Tring Running Club does have a number of fell runners, maybe 12 or 15 in total with a hard core of about 6. As mentioned we normally do the FRA relays and other fell relays and races, mountain marathons etc, but we may be too far from you and too far from the fells to be the right club to join. But don't let me dissuade you from coming along to club runs, you will be made welcome! Dark Peak could be a good fell club to join if you are committed to regular travelling, they seem to have quite a dispersed membership from what I hear. One of our members was a Dark Peak member when living in Sheffield and stayed a 2nd claim member after moving away.

Depends what you want out of a club though. There is massive benefit in joining a local club to go training with, not least having people to train with. You will also find that having people you know to compete against in local races is a big motivation to train harder, go out in bad weather etc. You may not get that by joining Dark Peak or even Tring. If I was you I'd join Fairlands Valley Spartans for training purposes. A road club for sure, but go for the training and the winter cross country league at least. You can still join DPFR for fell running and run for them in races, I doubt FVS are registered for fell running.

About navigation - you can always go and recce a few of the Peak District races, finding your way round will be a good nav exercise. There will doubtless be DPFR people keen to show you round, but that will unfortunately remove any any training value - find your own way first and then get them to show you the racing line! Fell Gazelle mentioned orienteering as a means of improving navigation and I can't agree more. I did this and got hooked on orienteering to the extent I stopped fell racing for several years, but now I have a much healthier O / fell balance