Packable Rain Jacket to accommodate larger upper arms ?

what's a Lands' End Package? Is that a new line?

I wouldn't bother with an umbrella. If you decide you want one, pick one up over there. A great tip I read on the Forum was to use a ball cap (something I don't normally wear) to prop up the hood of the rain jacket. In London in late September, I'm not sure a rain jacket is for "just in case." I would expect it to rain. Maybe look for a lightweight hooded jacket that is maybe only water repellent but comfortable to wear all the time and plan to duck indoors (or buy that umbrella) if it starts to pour. Something like this, that is soft and fairly stretchy. https://www.sierra.com/smartwool-smartloft-midlayer-hooded-jacket-merino-wool-insulated~p~9tnyw/?filterString=womens-jackets-and-coats~d~261%2F
As far as fitting your arms, I would try going up a size to a 1X or, depending on the rest of your shape, trying a men's jacket, as they are generally bigger in the shoulders. You could try a rain jacket with pit zips and see if that works. Or look for one with raglan sleeves, like this: https://www.sierra.com/eddie-bauer-port-townsend-rain-jacket-waterproof~p~8wmvx/?filterString=womens-rain-and-wind-jackets~d~599%2Feddie-bauer~b~1106%2F

The problem isn't your arms, the problem is designers who add length not circumference to the sleeves as the size goes up...whew, I've been waiting 40 years to vent. Thanks for that. I just buy 1X. I have the same Land's End Jacket and I recently bought a 1X Kuhl's Stretch Voyager Trench rain jacket. Kuhl's definitely runs smaller so the arms fit but the bulk of the overall jacket isn't oversized so it fits nicely. You have to order online because REI stores don't think women who wear 1X exist.

I have this rain jacket which can be zipped into its own pocket. I never actually do that as it is easier to just fold it up.
https://www.rei.com/product/227614/rei-co-op-rainier-rain-jacket-womens

It comes in a variety of sizes.

I see more plus sizes in my local REI than I used to, KD. At least they make and sell them, unlike so many other brands. And the return policy is very good, so it's easy enough to order several options. The prices, OTOH!

G3rryCee, perhaps in Bellingham, WA big arms and 1X are more accepted at REI, but in Southern California REI stores, not!

KD, that tracks.