I would like to establish the type of tenancy your tenant has. Can you please clarify.
A section 21 notice to quit relates to assured shorthold tenancies. Your notice could be invalid.
As to the locks, generally under a business tenancy this is the tenant's responsibility but must provide the landlord with keys if locks changed. Depends on the tenancy terms. If you change the lock you must provide the tenant with a key whilst it is still a tenant under the tenancy agreement. It will be a tenant until such times as the tenancy is determined (effluxion of time, surrender or forfeiture).
You cannot lock the tenant out unless the Tenancies been determined, a business tenancy can only be determined during the term by surrender or forfeiture, not by a s.21 notice.
As to a surrender, if the tenant vacates, hands back keys and you accept this then it is a surrender by conduct. The landlord does not have to accept.
I cannot comment on any potential claim/counterclaim from the tenant due to insufficient details and facts.
You have a potential claim of £2500 against the tenant which you can bring in the small claims court.
Any questions or queries? Happy to assist further.
I can review the tenancy and notice served by way of additional service, if you want advice on the documents.