Regular poster here, name changed because I don't feel comfortable posting without anonymity. I'm a GC mumsnetter.
I went for the first time I've ever been to Pride in London. I moved here X years ago & have been to Pride in other cities. I had a great time - the atmosphere was mostly just fun, people were way more relaxed than anybody in London usually is, and I came home wishing the world was just a better place.
I watched a fair bit of the march. I went to several stages with music acts on. Went around a few stalls. Had my picture taken with a couple of famous people, as you do. Did a lot of people watching. Went to a few bars. Drank some gin 
I've seen a few posts on here about a lot of fettishwear there and concerns about kids at pride seeing stuff that wasn't appropriate or indeed relevant to Pride. I hardly saw any leather and BDSM fetishwear, although I think that was perhaps because I didn't go to the areas that those things are more likely to be found. So, it certainly wasn't everywhere. I saw a few families with kids, and I don't think I saw the kids seeing anything in that way. I'm sure that stuff IS there, but it wasn't everywhere IYKWIM.
What struck me most was that the whole thing is tremendously skewed towards the T now. I know others on here have said Pride is now all about the lgbT and from what I saw I'd agree. By far the second most common flag I saw after the rainbow flag was the trans one. Lots of people wearing it, and what I thought was most weird was that often there would be a group in the march with rainbow flags, and the occasional trans flag dotted in that group.... but no others at all. I also don't think I saw a single lesbian, gay, bi or any other type of flag in the actual march apart from rainbow and trans ones. On individuals, yes, but not in the actual march. Of course it was huge and I didn't watch it all, so maybe it was just timing.
The other weird to me thing was that corporations and organisations who don't only serve the T, chose Trans flags to display instead of any rainbow ones. Obviously it's hugely commercial, but say, a high street bank or coffee shop would have a bus, float, or marchers, mainly to advertise but also to show that they are an inclusive employer... well, I cant remember specifics but some of them chose to do all trans flags and colours rather than rainbow. Most stuck to rainbow, but maybe 5-10% did trans only and maybe 30% had rainbow + trans and not others.
There is now a rainbow flag with black and brown added, to be inclusive of people of color. I saw several groups in the parade and around making the point that pride is centered on white people. The most commonly seen flag is still the one without those two colors.
I saw a lot of people with trans flags or colors on them. I haven't met many trans people, but seeing so many in one day made me notice how obvious there original sex always was. I'll try to get my language right. People of female sex have hips, and their facial expressions are softer. They're shorter. Even with no obvious chest they're female by silhouette. Male torsos don't have the same waist, and the bra lines you could see were in the wrong place and fitted wrong. Biological men are a different shape and they stand and move differently. Their voices, facial hair and bodies might have been shaped by taking hormones (but as a guess not many of them had surgery or hormones) but definitely not enough to pass. I now feel like I've seen first hand what not passing looks like. They don't. They just don't.
Body language - Young trans women were effeminate, curved shoulders, but hips and feet stood wider like a man. Young trans men standing arms close by sides and legs together. All the younger ones looking relaxed and happy to be there but not in their own skin - but I might be projecting.
Older trans men? I didn't see a single one. Maybe they pass better and don't go around with the flag, even at pride, but that's not my gut instinct. I don't think they were there.
Older trans women... well there were plenty of those. Some dressed conservatively in a longer dress and despite being at pride seemed nervous, keeping their chin tucked in like they were trying to hide. But most in bold patterns like flowers, leopard print, sequins, and skimpy or revealing outfits. Really high chunky heels. Fishnets. Bikinis. Big look at me eyelashes and make up colours. I mean, this was pride after all. They tended to stand with hips thrust forward, legs wide. Their walk was a striding male walk, even in heels, the type I'd associate with a hoodie wearing man at a football match. The type that as a woman seeing a male-dressed man walking like that I'd have clocked him a mile off and made sure to avoid eye contact, maybe changed my route. The type of body language that frankly I feel unsafe around.
They gave off an air that I felt uncomfortable around, totally different to the young trans people. I chatted to loads of people, that's just who I am and the type of day it was, and I felt happy with and warmed quickly to the young trans people the same as I did with anybody overtly out as lesbian, bi or gay or anybody who didn't advertise. Mostly the young trans people just seemed to be the same people I'd have hung around with and been myself as a teenager - rejecting gender stereotypes and finding themselves. I'm just really sad that it seems like this generation are identifying into a different gender and making physical changes to their bodies, because it just seems to me if only "gender bending" was normal to them like it was back in the day, where boys experimented with eyeliner and dresses and girls could wear t shirt and jeans and none of it was batted an eye lid at, then these young people would be happier in their own skin and not feeling like they were born in the wrong body :/
I hardly saw many young gay men - again perhaps I was just in a different part of London to them? 40+ year old male couples were around. No flags, a few t shirts.
I saw a few lesbians, young women mainly. Again no flags, a few t shirts. They seemed to be keeping a fairly low profile.
I want to talk about the stages and acts the most. I'll press post on this then add it as a comment.