Not sure you have right to it before the renewal date, but I've never had a problem getting one when I don't renew, change insurers and have to prove the NCB for the new insurer.
Not many insurers will provide proof before the renewal date, as there is still the chance of a claim that may affect the NCD entitlement.
It is also possible that the NCD entitlement would / could be used twice, particularly if you presented the NCD proof to a new lender for a different vehicle AND renewed your existing cover.
If you have protected NCD, that may offer a different perpective and would allow for a 'provisional notice of NCD entitlement'.