Well ok let's look at things like this, the seriousness of the offence matters only to those who it will affect. This is you and in this case any potential employer. If you don't need a crb for the line of work you do then the matter of the offence coming to light relies on two things. Them asking and you telling. If they don't ask don't tell. If you feel that you morally need to inform them then they need to then decide if A) the offence is such a serious level it will affect the job and B) if your upfront honesty balances things out.
If you need advice there is always Nacro who work to help people with a record but I feel that the main issue here is your concern. This to me says that your obviously remorsful for whatever it was and believe me that goes a long way when hiring folk. My advice is just take it as it comes, offer info that is asked for and let your skill at the job do the talking.
Punishments like fines,probation and community service are not looked on as being major anyway just custodial and even then the help available is immense so there is always light at the end of the tunnel.
I think remorse is now showing as fear and in turn worry at how this will ruin it all but trust me, the measure of a man(or woman) is shown in how they accept wqrong and move forward and I can tell you'll be ok. Try not to worry too much, take it as it comes. :)
