I did see the other post, and appreciated Debbie's timing (which must be coincidental). I, too, have had problems with agencies. There's one I signed up with in 2015, and I still haven't heard from them. Another was quite open (and polite) about not placing people with unspent convictions to disclose. A third actually had a session at my local Job Centre Plus, and were quite clear with them that they would accept people with unspent convictions. They didn't, though, not a single one. Unless it's one of the few who actually specialise in helping people with unspent convictions, it's a waste of time. If the Job Centre insist, then do what they say, safe in the knowledge that it's a waste of time in the sense of it not helping you to progress, but in another way it means you won't be sanctioned, so it's not a complete waste of time.
As Chris Stacey said: Although its not formally part of the sentence that is handed down in court, the criminal record that someone comes away with effectively becomes a second sentence, which can have a long-lasting, if not lifelong, impact.