The Straits of Dover are a deathtrap. Period. You are finding out why right now. If you have 7 destroyers on you and your battery is low, the ugly truth is that they have you and your current career will soon become part of the 75% Club. I may not be very nice in saying it that way, but there is no need to sugar coat it. Your boat is already dead.
If you have a save
before you got busted...
...reload and dive as deep as possible without running aground and make your way at silent speed
manually adjusted to 1 knot. Do not be afraid to stop to let task forces go by and resume your course when they go away. Take NO risks and surface
only after you have made it through. Do not get too close to Calais as there is usually some "fun" waiting for U-boats there, too. The only way through is straight up the middle.