It's an interesting question
@Mrob1982, but is it something that is likely to happen?
Is it likely that you will be pulled over and tested for drugs if you are not driving erratically? From what I have read the UK government has set the limits at very low levels (basically zero-tolerance but at levels set to account for "accidental exposure") for drugs such as cannabis, cocaine, etc, ie illegal use drugs, but the limits set for benzos (and methadone and morphine) are higher than normal prescribed doses and even then you have a legal defence if your limit is above the level set but your driving is not impaired.
I know what you are worried about but I also read that even if you have not been prescribed them you "could" be prosecuted if you drive with "certain levels" of these drugs in your body and have not been prescribed them. I would imagine these levels are quite high the same as if you have been prescribed them.
The laws were set on the basis of a "road safety risk based approach" to benzos and the like, ie not to say you cannot drive with them in your system at all. So I truly think you're worrying unnecessarily.
Benzos are widely prescribed here and it just doesn't seem to be an issue that arises.
You ask how long it is detectable via the test, and you are talking about possibly being tested days after use. I think even if there was any detected then, it would be minimal, to say the least. And the allowable levels are clearly set quite high. I just think they are more concerned about alcohol and illegal drugs such as cannabis etc.
Just don't get involved in a high-speed chase with the rozzers and I'm sure you'll have nothing whatsoever to worry about.