What to Wear

contact the hotel and ask them if they have a dress code, if not wear what you like.

I always pack one pair of black slacks and a nicer top. or a plain shirt and scarf. Also I pack a lands end tee shirt dress or polo dress - very comfortable for traveling and dresses up a bit when you want some polish. Dad brings one pair dark slacks and a collared shirt. Done.

My experience at posh and not-so-posh hotels throughout Europe is that casual tourists all tend to dress the same. Jeans are well on display in anything from a hostel to a 3-star spa hotel. The only real difference you see in clothing is among business travelers or around dinner time and/or cocktail hour, when guests at the higher-end establishments tend to dress a little fancier. But this could still include jeans.

Thanks you for your reply and advice!!

I take polo type shirts and Dockers; comfortable, light, but a little dressier.

Traveling light doesn't necessarily mean that you must wear jeans. I don't get that connection.

To travel light, you may not want to take jeans at all. You might wear a nice pair of black pants (could be knit for comfort), white blouse, colorful scarf. Likewise, you could take along a pair of tailored or knit slacks in tan, khaki or chocolate with a couple of tops to match. Make sure everything will mix and match. Some people here limit their pieces to a palette of black, gray, white plus colorful scarves.

The theory behind traveling light is not necessarily to wear jeans all the time. It's simply to limit the number of pieces you take. And those can be dressy if you want them to be.

As others have said, nice jeans are seen on tourists and locals around London.

My husband and I went to London last May. We were there on May 5th, the date you say you will be there, and it was hot as heck.
Much too hot to wear jeans. I was glad I had brought a pair of nice capri pants in a cotton-linen blend, a sleeveless white blouse and sandals to wear. This is what I wore last spring to visit the Tower of London.

My guess is for part of your trip it will be too warm to wear jeans, and for part of your trip it will be cooler and just right for jeans.
The temperature varies from region to region in England, and varies from day to day in London.
Several days after the hot day in London, we were in the Cotswolds and I was wearing black knit pants and a fleece pullover.
The day after that, I was in Stratford-Upon-Avon, and wore the black pants with a white silk blouse and a long string of pearls to go to a nice restaurant for dinner.
Black knit pants are the most useful item to take to London or England, in my opinion, not jeans. But that's just my personal preference.
Sometimes I take jeans in addition to the black pants.

Rebecca's answer is great, but I just want to emphasize that jeans are heavy and they take a long time to dry, so they're not the best thing to bring, regardless of whether or not they're dressy enough for a particular situation.

Check to see if there is a dress code. Some hotels do not allow t-shirts and shorts in the lobby in the evening. (Ritz London). These are usually 5 star properties.

Thanks everyone for your response and advice!!

Okay, what about leggings or yoga pants? They are so comfortable and would probably wash easily. Are they too casual?

No yoga pants or leggings outside your hotel room.

Normally I would agree with Leslie.

If the yoga pants or leggings are new and look nice, you could wear them if paired with a tunic that comes down far enough to cover your backside.

Leggings with tunics:
https://www.softsurroundings.com/P/28125/?suggest=2
https://www.softsurroundings.com/P/29245/

I've used those pages/pictures as examples of what would look presentable and even very nice.
I think it's all a matter of dressing up the leggings with a cute top that is long enough,
so you're not showing a really tight fit of the leggings in the backside.
The tunic covers that problem.
I prefer knit tunics for comfort.

And yes, I would wear yoga pants or leggings on your flight over and back.
You might as well be comfortable on a long flight.
Nobody will look twice at what you are wearing on a flight.

I'm not a fan of leggings. I do like stretchy pants, but knit ones are often too heavy.

For my next trip, 4+ weeks in Italy in the summer, I plan to have three pairs of pants. These are the ones I plan to take/wear. When I test-pack soon for a domestic trip, I may decide to cut back to a total of 2.

Classic Fit Slim-Sation Pull-On Straight Leg navy pants. I normally wear an 18, but had to return them for a size 16. According to my kitchen scale, they weigh 12 oz.

Classic Fit Slim-Sation Straight Leg jeans. Same size for these. Finally some stretch jeans that look decent. My scale says they weigh 16 oz. I don't find jeans to be too hot or heavy. These will likely be on my butt much of the time. They are dark blue and look great.

WinterSilks Silk Linen Easy-On midnight navy pants. I got XL. The waist is very stretchy and a little big. That may be useful, especially toward the end of my trip. They weigh 10 oz.

I think any of these bottoms would go anywhere with any of the few tops I plan to take. I've test washed and hung dried them and the jeans do take about 6 hours longer to dry than the other pants.

I also have a navy pair of these FlyAway "travel" pants. They weigh 12 oz, can be rolled up and secured with a tab, and have all the pockets and zippers that usually are a part of these kinds of pants. Considering how many people I see in jeans everywhere and saw in London last year, I would feel far more comfortable wearing jeans than "travel" pants at a nicer place. I'm more likely to wear these pants at home than while traveling.

As others have said, the weather is variable. I arrived at Heathrow on 2 May last year and went straight to Bath. It was somewhat sunny, but not warm. In fact, it was not warm anywhere I went until I finished the trip with a week in London 5-12 June. The weather was much like Seattle: cold, wet, and windy.

I agree that leggings are just fine. Just apply some common sense and (age appropriate) taste. I making my 18 or 19th trip to England this September and I have never taken a pair of jeans. I know. They mix and match with everything. So does black.