Great Britain had one if the world's most powerful navies during the Great War, so why didn't they ever attempt a landing of some kind behind the German lines instead of keeping their attention focused on the trenches of the Western Front?
^ this answer I gave a while back should be pertinent
The reason is that: A) The bulk of Germany's armies were on the Western Front, and sending troops off to an area that would be difficult to resupply would only play to the Germans strength, allowing them to shift divisions from elsewhere to meet this new threat. And B), this would involve sending British warships, many carrying troops, through the Kattegat and Skaggerack straits, close to German waters in fact, essentially the straits of jutland, where the German Navy could bottle them up and have a field day shooting at them. And don't forget U-boats and minefields. Then, they've got to land them along the German occupied Baltic Coast, and keep them supplied that way. It was simply too risky, and highly undesirable when the British and French could focus their forces closer to home on the Western Front.
There were plans for a corps-sized landing near Ostend to coincide with a breakout from the Ypres Salient in 1917, but poor weather and German resistance precluded the breakout.
Will also pipe in that my answer in the same linked thread may be fo interest.
Fisher and Churchill were essentially OBSESSED with the very idea, even getting pledges to use a Russian Army Corps.
But nothing could be done until the threat of the High Seas Fleet was dealt with, and they just would not oblige the RN by coming out to be sunk. And most of the leadership thought the alternative coastal landing ideas were between foolhardy and suicidal, in particular Jellicoe was firmly against any of it as it drew resources away from his fleet.