Why were British coastal offensives repeatedly deferred?

Why was a British and Belgian advance along the coast repeatedly deferred?

British planners wanted that axis, but French requests for relief redirected British attacks toward threatened sectors.

British planners did want to attack along the coast and repeatedly prepared versions of that operation. French pressure elsewhere on the front led British politicians and commanders to shift attacks toward threatened sectors in hopes of drawing off German troops. The coast remained an objective, but coalition needs kept displacing it. During the final hundred days, British forces advanced through flatter coastal areas with greater success. Allied politics could redirect an operational preference even when the geography behind it remained attractive.