You cannot get a CRB check through yourself. CRB checks can only be given to employers from an industry/business. You can apply for a police record check under the Freedom of Data and Information act. This is not a CRB check. It cost 35 pound. It is the same check that you must do to gain a Visa to America.
If all your convictions are spent, there is no need to worry, you don't have to declare anything. It is also illegal for any employer to reject you because of previous convictions. In the armys case I know myself that you "can" get accepted entry with an unspent conviction, I am living proof of this.
This country gives people who have been convicted of a crime a Rehabilitation period. Once that Rehabilitation is completed it no longer has to be declared to an employer. There is no 7 year period as previously stated. The most common rehabilitation time is 5 years. This is because of orders such as community service, probation & fines
etc. They all require a 5 year rehabilitation period. Under 18 months in Jail requires 7 years. Over 2 & 1/2 years in jail is 10 years and never comes of your record.
I commited an Affray when I was 15 years old. Under English Law the police must remove this completley from the database by the time I'm 25, 10 years from the date of conviction. If you commit any serious crime when you are over the age of 18 it stays on the database for 20 years.