To the British Heligoland etc should be considered a no go area,and losses should reflect this if attempted.
To be fair, the Germans didn't mine it off until about 3 months into the war, prior to that, the British ran a couple of sweeps through the bight, and managed to provoke at least one sortie that they spanked (Battle of Heigoland ... think the germans lost 3 or 4 cruisers and some torpedo boats).
Declaring it no-go strikes me as a bit extreme, but modelling minefields as static attrition would make mroe sense e.g.
Starting on Turn 6, declare all hexes within the heigoland bight to be minefields.
Any cruiser counter entering a minefield takes 1D2 strength point losses.
Any BB counter entering a minefield takes 1D4 strength point losses (heavy units are actually move vulnerable to mines in most cases)
if you want to really get fancy with it you could allow cruisers to 'sweep' mines and eventually reduce their strength. There *were* historical attempts to force hostile minefields with mine sweeping under fire (the british tried, and ultimately failed, the sweep and clear a minefield in teh dardanelles during the gallipolli campaign, but they did make progress before they abandoned the effort).
I do not think anywhere should be a no-go - so long as there is an appropriate chance of loss if a player chooses to risk entering certain areas.
22nd November 1944 - The British Pacific Fleet is born (temporary avatar changes to commemorate the ships and aircraft). HM Ships Formidable, Illustrious, Victorious