Remote indication and status display
It is an ICAO requirement for the approach controller to have the status of all approach navigation aids displayed in a relevant position.
It is usually the approach controller (procedural or radar) that gives the clearance to use the approach aid, not the tower controller, as the ratings are different.
It is also a requirement for the tower controller to have the status displayed.
Use of the navigation aid outside published hours is prohibited in England, Wales and Northern Ireland under the conditions mentioned. You require an ATC clearance to use the navigation aid, which you cannot have if the aerodrome is closed.
There are certain aerodromes in Scotland where use of navigation aids for instrument approaches is allowed, and no ATC service is provided. However, this is a very very grey area of the law, and restricted to a few aircraft operators only, mainly Loganair.
Interestingly enough, use of navigation aids to approach many offshore installations might be illegal as well! Nice to see that SRG and DAP cannot agree on this, but the "Authority speaks with one voice".
Interesting interpretations would occur when using en-route navigation aids, based at an aerodrome that is either closed, or does not have an ATC service. Not that I am suggesting that anyone still has the old plates for Goodwood, for example.....
There is an interesting exception to the Tower ATC function which occurs at Lasham. There is approach control, but no tower ATC, from memory at least, please correct if wrong! They give you an SRA to 2 nm and then it is up to you.
The controllers have "remote indication" in their respective dark holes or greenhouses. The equipment has monitoring provided with it. Most modern navigation aids will shut down automatically if the system is close to the tolerance boundaries. What you will not notice is some distortion of ILS signals if only a near field monitor is in use and then someone parks a 747 in front of the remaining GP signal. That will make you fly at about one degree, not three. Many navigation aids have two channels, so that if the A system fails, the B takes over. In the event of a major problem, or single source failure point, then the whole lot should stop transmitting.
Remember, that you need the identity, not TST for those who cannot interpret morse!
Remember about ASSOCIATION of identity signals as well. The group of four identity tones for a VOR/DME. Now, which way round is it... three for the VOR and one for the DME.... well, you should know.... if only listen for one then it does not tell you all you need to know.
As for the opposite direction stuff, well fine for VOR or NDB, but the LOC signals would interfere with one another, so they are interlocked out so only one end can transmit at once.
I guess in summary, for flying in the UK, only during published hours, with ATC clearance!