Your question can be read one of two ways. Either you mean "Can an applicant with an unspent conviction not declare it and get away with it?" or you mean "If an employee has a spent conviction and is not obligated to declare it, is there anything in place to prevent him/her from declaring it and/or anything in place to ensure that the potential employer can be forced to disregard it?"
If the former:
The short version - in non-high-risk jobs, it depends if the employer chooses to carry out checks (if no, getting away with it is easy. If yes, getting away with it is not so easy). The long version - In high-risk roles, it should not be possible. My understanding of it (and remember, IANAL) is that if you are explicitly asked about convictions on a normal (i.e. for a non-high-risk role) application, you only have to declare "unspent" convictions. If your employer then applies for a criminal records check (which, as I understand it, any employer can do with the individual's consent) it will only return "unspent" convictions. "Spent" convictions remain on record. They are not expunged. If you are applying for a higher-risk role, your employer may have the right to request a more complete criminal records check, under which circumstances ALL records are returned, "spent" and "unspent". Indeed, if you are applying for a high-risk role, my understanding is that the application form MUST make clear that you need to declare both "spent" and "unspent" convictions and that (unlike in a non-high-risk role, where the criminal records check is optional) the company MUST have you vetted by the DBS (Disclosure & Barring Service. I believe that this replaces the CRB - Criminal Records Bureau). In low risk roles, however, because formal checks are optional, it has the potential to be relatively easy.
If the latter:
I'm not aware of anything being in place, other than human nature of a person not normally wanting to portray themselves in a negative light, to prevent an applicant from declaring spent convictions that (s)he does not need to. That being said, if (s)he does declare it, depending on how much detail (s)he gives, the employer should be able to determine for themselves whether a conviction is spent (the length of time it takes for a conviction to be spent is, I think, determined by the length of the sentence. Therefore, provided the applicant can give accurate dates, the employer should be able to do the math without needing so much as a calculator). The downside it that, whilst the employer should be able to determine that the conviction is spent, and thus should be disregarded, I am not aware of anything in place to ensure that once they have determined such that they do disregard it. Indeed, it is likely that, should an applicant slip up and disclose a spent conviction when there is no duty to do so, the employer will take it into consideration and choose an alternative candidate but, when pressed for a reason, will not admit to this being it (as can be fairly common with ageism and other forms of discrimination).
As for the question of whether the employer is allowed to ask, only when hiring for high-risk roles are they permitted to ask about spent convictions. I am not sure what the legal options are for an applicant with spent convictions who is asked about whether (s)he has any by an employer who is not afforded such an exemption. I am not even sure whether they could be found guilty of fraud if they lied and didn't disclose them as, as far as the employers legitimate entitlement to information is concerned, the spent convictions do not exist. I'm also not sure whether there are any sanctions laid down that can be imposed on employers who ask when they do not have such an exemption. Maybe a real lawyer would like to chip in here and fill in the blanks?
As for being expunged from public records, the answer is no, as they still need to be declared for high-risk roles. My understanding is that they cannot, however, be referred to in most court proceedings. I also believe that it is an offence to publish details of spent convictions for malicious purposes. I am not aware, however, whether this means that archived/historic publication of such convictions made whilst the conviction was unspent are required to be purged (I doubt it).
Again, IANAL, and I look forward to any clarification that can be made by a real lawyer.