I vote cobalt blue, gorgeous on you. You really don't need to think about anything else. That trenchcoat absolutely works.

I swear by London Fog for the UK, but my one had a zip in liner and of course was ankle length, lol. The cobalt colour is super fantastic on you — and you say it’s more substantial? I’d say go with that one. I find England to be like SF, it’s not really that cold and yet…

It will rain on you and black is good for London. Take an old black one with you If you are packing in a carry on remember that the trench may also be your robe and must be water proof. You will live in it. Take a cashmere sweater that will fit over other tops you have packed. It may be cold. Take or wear short boots. It helps if they are waterproof. You will incounter big puddles crossing streets.

Blue gets my vote too! Love it on you!

I love the blue on you too . I am currently on the hunt for a raincoat with a hood and some kind of zip in lining , it's proving hard to find as I would like a bright color.!

Your trenches are all fab cut and color(the cobalt!!!wow!!).
That been said I don't know if it's 100% true but speaking about weather in UK I am always reminded that end of May there is already kind of summer-in terms of girls wearing bare legs under skirts and dresses as early as possible! So do you really need a trench? I hooded raincoat/light packable puffer over the other jackets you'll bring will probably also do. And maybe even lighter, easy layers for under, too.
Still I very much understand the need of a lighter trenchcoat---I longed for the perfect one for more than a decade (as I too had my perfect version in black at first). A girl needs options and I am convinced a lighter one will earn it's money back. Mine is now a saturated mid beige (makes a lot of difference to the paler pastel beiges I tried in the past) and I love wearing it over my darks a lot (usually black sheers with skirts and dresses) or dark cigarette pants.
And, Angie is so right- it doesn't have to be oversized- little under knee length is more functional IMHO for the warmer weather(and a hood would be great of course). Maybe you'll find one for yourself in London!

More love for cobalt! Black will feel dark in May (ditto -- I think -- the tweed jacket you're thinking about), when the sun can be quite strong, even if it's still coolish, and there can be cold days. You will be unlucky if you have lots of heavy rain. Have a wonderful trip!

Ok. I appreciate all the input here -- you have all given thoughtful comments.

I will be watching the weather and taking hte cobalt coat (most probably). I agree that weather can be getting warmer in mid to late May and I may have no need for a coat at all -- but just in case, and especially for evenings, which are much cooler -- better to be prepared. As the time gets closer I will prepare a travel capsule. I'm thinking it might be similar to the one I took for San Francisco last August. But we'll see!