RNav has been mandatory in the UK above FL100 for quite some time, and is now (or very soon, I can't remember the exact date) mandatory for almost all UK airways. The reference is in AIP ENR, somewhere near the beginning. If I get time later on, I'll try to dig it out unless someone beats me to it.
I'm not familiar with Czech rules, but this is the way IFR flying is going in the UK, and presumably worldwide.
FFF
----------