The typical asking rate per listing is in the $20's per square foot per year. The Hampstead submarket is seeing a a boost in rates. In the last 3 months, the median rate went up $25 per sqft per year.
In the last 90 days, 2 listings have been listed, which means leases are probably leasing quickly. You'll want to be prepared to make an offer immediately. There aren't many listings below 2,000 square feet available, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 1,980 sqft.