The only hard time I've really had in the Channel was in U-48 ages ago before I had Grey Wolves. I was going into Dover and there was a large cargo ship leaving port. So I fired a torpedo and broke its back. The armed trawler that was escorting it somehow found my location and called for reinforcements.
*20 in-game minutes later* 9 destroyers at my location. Yes, 9. It was madness. I found it to be impossible to escape. U-48 went missing for the rest of the war.
U-123 also encountered large numbers of destroyers after an attack on Portsmouth, which was a terrible idea anyways. By the way, my sig is getting out of date, I think I need to update it. The prize was worth it though; 3 tankers sunk. I may have even passed the wreck of U-48 on the way through the channel; who knows?