Can I use any airport's ILS in uncontrolled airspace for practice without contacting ATC, and break off the approach before entering the ATZ?
Legally yes (I think, Class G) but it would be stupid to do so because - in the UK - the unit there is highly unlikely to have any radar coverage.
Similarly can I use any airport's VOR or NDB, and practice a few holds while keeping above the top of the ATZ?
Again yes, in Class G, if you are above the ATZ (2000ft AGL) but again it would be stupid, for the same reason. But using these navaids at a suitable distance is obviously fine. OTOH why bother, when there are plenty of VORs and NDBs which are not at airports and you can just pretend they are? Plenty of people have practiced VOR procedures around Seaford for example. Obviously you want to do it away from GA traffic (which means well above 2000ft
) and as high as possible, and in VMC and keeping a lookout.