Or you can just not replace the flaked barley with anything. Just up the pale malt by an amount equal to the missing flaked barley to keep the OG up.
Flaked barley is a fairly recent addition (last 100 or so years, it was illegal in UK beer for most of the 19th century) so it would be interesting to see if you can tell the difference. Flaked barley is supposed to dry out the beer a bit, improve the foam and head (high protein content) and increase body (beta-glucans, a glucose polymer gum not a protein as might be thought, and a known culprit for encouraging stuck mashes), but the cynical will say flaked barley (cattle feed!) is just cheaper for the breweries.