I'm doing my first TA cruise in April also. However, I'm leaving out of Miami and ending in Italy. Per Yahoo weather in April the average low for Fort Lauderdale is 66 and the high is 82 with 3.91 inches of rainfall, in London, England the average low is 38 and the low is 54 with 2.10 inches of rain fall. London's high is lower then FT. Lauderdale's low . My suggestion is to bring layers. Also I decided to bring convertable pants they convert to shorts. That way I only need one pair of pants. Here is a men's pair I found.
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I like to pack light, but the right answer to your question is "all of the above." And think layers. You'll obviously start off warm and end up coolish and very possibly rainy.
If your goal is to get the maximum deck time in the warmer weather earlier in the cruise, you have to think tropical/semi-tropical. By the time you get to England you could well be in sweaters, windbreakers and rain gear. How you balance this out will have a lot to do with what you're doing both pre and post cruise. I hope you're not just flying home from England as soon as you get there.
Anything made of Thinsulate or its equivalents might be a good idea.