All postal operators in the UK have to comply with the Mail Integrity Code to keep their licence:
"a requirement for prospective Relevant Employees to declare any criminal convictions or any cautions or conditional discharges for offences relating to:
(i) Postal Packets; or
(ii) dishonest conduct generally (in particular, theft, obtaining property by deception or fraud)
and guidelines on how any such convictions, cautions or conditional discharges declared by prospective Relevant Employees will be taken into consideration in deciding whether or not to employ the prospective Relevant Employee."
http://stakeholders.ofcom.org.uk/binaries/post/341.pdf" onclick="window.open(this.href);return false;
This requirement forces postal operators to ask employees to disclose information that they don't have to disclose (Rehabilitation of Offenders Act 1974). Providing a postal service is not something that is exempt from the ROA so postal operators may only ask for information that is revealed by a basic disclosure. However, technically it is not against the law for postal operators to ask people to reveal information they don't have to reveal even though this makes postal operators appear deceitful.
Anyone with a spent criminal conviction who wants to infiltrate a postal company to do crime would be aware that postal operators can only ask you to do a Basic Disclosure and the infiltrator would know they don't have to reveal their spent convictions regardless of the wording on the form. The questions will not catch out someone with criminal intent, they will only make it less likely that people with spent convictions find it harder to be rehabilitated and get treated the same as everyone else in society.
There is no reason why your employer should know you are a retired cannabis farmer because your conviction is spent.
"Cautions (including reprimands and warnings) are covered by the Rehabilitation of Offenders Act 1974 so will become spent immediately (apart from conditional cautions which will become spent after 3 months). This means that if you are asked on an application form if you have a caution you can reply 'no'. For conditional cautions it would be after 3 months since the caution was issued, up until that time you would have to reply 'yes'."
https://www.askthe.police.uk/content/q562.htm" onclick="window.open(this.href);return false;