You're welcome, Nestor. There is information about the new sentencing rules on the Unlock website here
, but your case seems to come down to whether your offence was committed on or after 1st February 2015. If it was, then they apply, if it wasn't then they don't. Of course, if they're not sure, that would cause confusion. If you have more than one offence, and at least one was on or after the relevant date, then the new rules apply.
The problem with the Job Centre is a strange one. They can insist that you actively seek work and that you keep applying for jobs, but not having a reasonable chance of finding a job shouldn't come into it. There are plenty of UK nationals who come out of prison and have no reasonable chance of finding a job, but so long as they follow the rules they are paid. If you are on Universal Credit, the housing benefit should be part of the same claim. If you are on Jobseekers Allowance, the housing benefit depends on your income and assets and is processed by your local authority - the local council. If either of them is treating you differently as a result of your nationality that is against the law. According to the Immigration Rules here
, you can only be deported after conviction if the judge recommends it. That sounds like you'd need to do something much more serious than your earlier conviction.
There have been suggestions in the media that the French don't extradite French citizens under the European Arrest Warrant system, but I don't know whether that is actually true. The official procedures allow extradition with no mention of nationality, so it could be risky.
It's probably better to stick to the line about the benefit to reducing re-offending. If there's no point keeping you here any longer, then a Senior Probation Officer has the authority to allow you to leave.
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.