A practical starting point is to research publishers and gift/postcard companies through directories such as the Writers’ & Artists’ Yearbook, which can list companies, what they publish, and submission guidelines.

It may also be worth approaching potential end users directly rather than only looking for large licensing agencies. For your subject matter, that could include local tourist attractions, charities, and businesses that may want London imagery for location-specific campaigns or products.

Another route is to test the market yourself with small postcard print runs using print-on-demand or short-run printers, then sell through local outlets, markets, or tourist-focused shops. That can help you build a sales track record and show demand for your images.

If you can demonstrate that your work already sells, you may have a stronger case when approaching larger postcard or gift companies directly for licensing discussions.