Your councils are wrong - 100%. There are two types of grant:
1) a SBRR Grant - £10,000 grant available to ANY small business that occupies a business premises in England with a rateable value of less than £15,000 and in receipt of SBRR.
2) a Retail Grant - £10,000 grant available to Retail, Hospitality and Leisure businesses that occupies a business premises in England with a rateable value of less than £15,000 OR £25,000 available to Retail, Hospitality and Leisure businesses that occupies a business premises in England with a rateable value of more than £15,000 but less than £51,000.
For the Retail Grant, your premises MUST be classified as retail/shop on your business rates bill.
There is ZERO exclusions in the government guidance that stipulates an 'office' premises cannot receive the grant.
In your shoes, I would challenge this with the council, point them in the direction of the government guidance and if needed escalate it to your local MP.