Could Britain have treated Heligoland as another Gibraltar?

Could Britain have regarded Heligoland as the next Gibraltar?

Yes, but only if it had acquired the strategic importance and fortifications that made it a permanent British stronghold rather than an idle possession.

The answer makes British retention conditional on a change of strategic outlook. Britain historically left Heligoland comparatively undeveloped because France did not sustain the northern threat for which the island had been acquired. If London instead treated the island as a North Sea equivalent of Gibraltar, fortification would bind it into British grand strategy and public expectations. That permanent political and military commitment would make a later territorial bargain with Germany far less likely.