Just a little point of guidance - a "spent" offence is not the same as a
"pardon" in UK law.
A "spent" offence is one which, owing to its age and severity, need not
be declared as a matter of course on MOST job applications, etc. It
isn't a pardon, its still a conviction/caution.
For immigration purposes, and even for tourist visas, ALL offences
should be declared, even those which are normally considered "spent".
I can't advise you on obtaining a pardon, except to say that I think you
should get advice from a solicitor or migration agent.
Pollyana
This is not meant to be legally binding advice of any sort, and you are
advised to speak to someone trained in this field.