OBK
Yep - bang on with your RNAV method.
Lat/Long is correct except the numbers come first followed by the letter eg - 4620N07805W.
Both are perfectly acceptable for flight plan submission. Ignore those comments regarding filing points 'close' to your routing....not a good idea. ATC Do give a monkeys and you could end up routing yourself into some controlled airspace by accident.
RNAV is a different kettle of fish compared to IFR reporting points, and the method explained above for filing is direct from CAP694.
Regarding ATC transmission, I'd just cross-reference your RNAV point on your chart - ie - two miles west of x, or five miles south of y....this is sufficient for transmission. Only read off the radial/distance/bearing if ATC require clarification, ie- they are unsure!
Hope this helps...........