I sympathise with your position, it's obviously very difficult.
As I am sure you are aware, you are bound by the terms of the Lease that you signed with the Landlord (I assume this to be Punch). You should therefore refer to the Lease and check to see if contains any break clauses. These are clauses which specify when you are able to terminate the Lease upon serving the requisite notice under the Clause. If there is no such clause then the only way in which you may released from the Lease is by the lease Term expiring, but plainly if there is a significant period of time remaining until the term expires then it will not be of much use to you.
Your other option is to check the lease for the "alienation" clause. This clause specifies whether you are permitted to assign (ie. sell) the lease to another person and in what circumstances. You will need the Landlord's consent for this and the clause will likely say what the Landlord will require in order for them to grant their consent. They may very well require the incoming Tenant to provide with references (eg. bank, trade and accountants), guarantors and also that you enter in to an Authorised Guarantee Agreement (if the lease was signed after 1st January 1996). Check the clause but it will almost certainly require that you are up to date with your rent payments (and that there are no other outstanding breaches of the Lease terms).
If you enter in to an authorise guarantee agreement you will guarantee the liabilities of the incoming tenant (if the incoming tenant defaults the Landlord will be able to come after you for the sums owed).
Were you to assign the Lease to an incoming tenant and then that person in turn assigns it to another then you would be released from you liability under the authorised guarantee agreement.
You should also check to see if the Lease allows you to sublet it. This will also be contained in the "alienation" clause referred to above.
You could contact your BRM and ask if he is able to refer you to a commercial agent who commonly deals with pub lease transactions. He will then market the Lease in attempt to find you a suitable incoming tenant. Failing that you should search online for commercial agent in the local area.
If you were to simply post the keys through the letter box then the Lease would continue regardless and you would still be liable for the rent until the Landlord forfeits the Lease because of your default and recovers possession of the property. Doubtless he would then sue you for the outstanding money that you owe under the Lease (or possibly take your personal property as distress for rent). It is better to attempt to deal with the Landlord than to bury your head in the sand I'm afraid.
If your reported the theft to the police and your former manager was charged and convicted then you may be able to obtain compensation. This would seem to be some way off in the future of course.
I am sure this is difficult reading for you but I hope you can acknowledge there is little reason in my sugar-coating you position.
However, I hope this is useful to you. If so, please kindly click "accept". Feel free to ask any follow up questions.
I wish you the best of luck.