Packing for London next week!

It's pretty nice here right now by current US Midwest and UK standards, in my opinion.

I would dress in layers. When the sun goes away it's a bit damp and chilly, but
the sun has been coming out at times each day and then it's pretty darned warm.
For your specific question, I'd say raincoat, but YMMV.

Leggings are in play here, by my admittedly limited observation skills.

I’d suggest the good quality raincoat with removable warm layers underneath, as the weather is very variable but not that cold at the moment. There will be rain, but probably not all day, every day, and often it will only be light drizzle rather than a downpour.

I would take a waterproof raincoat ( not water resistant) and pack layers. Maybe a lightweight jacket like a Patagonia nano puff. Also you could pack Cuddle Duds to layer under clothes if it’s cold - they are very lightweight. Maybe a Merino Wool cardigan. I would take a pair of jeans and a couple of black pants. Maybe some water resistant boots and a pair of sneakers for lots of walking. Allbirds has water resistant sneakers that are very comfortable.

We were just there. We arrived Wednesday afternoon to cold and rain, had a very nice day on Thursday, and cool drizzle on Friday morning when we departed. My lined Eddie Bauer raincoat was perfect for all conditions. I just added an extra layer underneath when it was cold.

I agree with what everyone else has said. Layers and a good quality waterproof jacket. A fleece layer and a scarf are always handy. It’s not very cold here at the moment and the weather looks to be similar for the next couple of weeks. It is unlikely to rain all day. You do get hot in London when going on off the tube so a very warm coat will be overkill. A lightweight puffa is perfect for the temperature but most of them aren’t waterproof unfortunately.

You can definitely wear leggings if you want to. London is incredibly diverse and you will see people wearing literally any fashion you can imagine.

Nobody knows what the weather will be next week, or tomorrow.

No, you can’t predict the rainfall accurately but I’d feel fairly confident that if 12 degrees is predicted everyday we are not going to be having a heatwave or a snow storm. It’s going to be on the cool side of temperate.