Up shit creek!
Yes, metabolites for deca are detectable for a long time after use - up to a year in some cases. It's the number one for drug tests.
As for your police application are you applying within Canada or England? I don't know what the English do. As for Canadian forces you will be asked about your drug history, and depending on which police force you are applying to, you will also be subjected to a polygraph session (lie detector). If you are caught lying to police within an interview, you will be banished to the netherworlds ( ok going a bit far-but you will never be allowed to re-apply) on grounds that you lack integrity. If you do not think you can beat a polygraph, which most can't, it may be better to wait a lengthy period (3-5 years) while remaining clean so as not to fuck your chances for life and own up to the fact that you used, that it was stupid, yadah yadah yadah, and will never use again. Once you're hired and off probationary period, then you're in the clear. If you go through your interview now, and choose to admit the use and it was stupid etc, you will likely be deferred for a period of about the same time (5 years), but will probably be able to re-apply.
Now as for drug testing, I don't believe that they screen for drugs at this time. You will have to go through a thorough medical examination, eyes, ears, physical (prostate, lube up and bend over) and some blood work, but I don't think they are looking for drugs on these tests, just for your medical condition.
Whatever you decide, good luck with it. Good money in policing, could be a good career.
