A suit of plate mail armour and carry a rubber chicken… (Monty Python)
Or a leather motorcycle suit and mirror-visored full-face helmet (The Stig)
I’m a Londoner. Tell her to wear what she likes although she might like to smarten up a little for somewhere like the Orangery. I mostly wear jeans and converse though if I go out in my sandals nobody bats an eyelid even at this time of year.
The one thing she might want to watch out for is the rain. It’s been particularly wet and cool the last few weeks and so she may want to bring a couple of extra changes of clothing or some waterproofs. Of course there’s no guarantee that March won’t be mild and sunny but personally I’m not going to bet on it. Oh and sod the ‘don’t wear bright colours’ thing. It’s perfectly acceptable here and an excellent way to find lost members of the group if you know they are in bright red or pink or something. 
Also, if she’s going to use public transport a) expect it to be more expensive than wherever you are and b) the first thing she does is go to a ticket office in a tube station and buy a Pay-as-you-go Oyster card. It’s cheaper than paper tickets.
If you think the “saloon” is the only place to see drunk people in London, you’d be very, very surprised by the reality of it.
Which reminds me - if you want to blend in with the locals, wear a dirty tracksuit and carry an open can of Special Brew. No one will even look at you. Or stand near you.
- They expect to eat the same food as they enjoy back home
- They loudly and disfavourably compare things (including the alien food they asked for) to the version they can get back home
- Maps. Unfolded. Walking.
(I’m not, of course, wishing to imply that all or even most tourists of any nationality, in any location, do this - merely that these things will make you stand out if you do them)
Re footwear in London, it struck me the other day as I walked through Waterloo station that everybody is wearing Uggs. Still.
If you are a woman and in London right now, there is an approximately 70% chance you have faux sheepshin wrapped around your calves.
For men who want something comfortable to wear around town and not stand out, I’d suggest something like Timberland ankle boots. Trainers/sneakers are fine, but not many people wear those big puffy white ones that Americans like. Also from a practical point of view, springtime in London often involves sploshing your way through some pretty deep puddles, so I would advise footwear that is at least vaguely waterproof.
I’m a mapoholic, but I would NEVER walk around anywhere with an unfolded sheet map in my hand and definitely not in London. If I happen to very discreetly take out my A-Z to check something I am guaranteed to be surrounded by people asking “Can I help you?” when in most cases I know my way around better the many natives.
Some of these are ridiculous. Brits don’t wear trainers, waterproof jackets or don hats? Yeah, apart from the masses of British people who actually do wear these items of clothing (especially if it’s raining or cold). Wear whatever you damn well please. Seriously, I’ve never understood the angst that Americans (especially) feel about fitting in: you’re a tourist, you’re here for two weeks at the most, what the hell do you care what somebody you’ll never see again, who lives thousands of miles away from you, thinks about your clothing?
Ironically, two things that British tourists abroad are currently known for (and not without at least a little justification).
Well, I’ve been thinking about moving back, and I suppose this decides it. My people obviously need me.
'Course, if the other 30% are wearing Crocs (not likely in the UK in March) they’re beyond help.
Back home, we frequently hear that we Americans are offensively garish and boorish when we go abroad as tourists. I suspect that many travelers try to bend over backwards not to embody that stereeotype.
And as a result they all end up looking like each other anyway.
In the US from what I see only teenage or college girls wear Uggs. Do adult women wear them in England?
I’m a little disappointed in the Brits here, setting up your wife to be embarassed by saying she’ll “fit in” by basically dressing like an American. It’s to be expected though, the British don’t want Americans to fit in. I’ve been to London, if your wife wants blend in, she’ll need to dress like this
It’s much more difficult for an male American, of course, it’s pretty hard to pull off the typical British man with clothing alone.
Younger ones, certainly. But the fashions for 20-somethings tend towards the unattractive anyway - Uggs, leggings, ridiculously skinny jeans that look like they must be cutting off blood flow…
Me, I’m in my 30s, and I like a nice frock or pencil skirt with a reasonable heel. I was singing on Friday at a club known to attract the local bright young things, and before leaving I twirled around in front of my husband and asked if I looked cool.
He hesitated, and then said: “No. But you do look beautiful. At our age we’re past the point of looking cool, so it’s best just to wear something that suits you and doesn’t make you look like you’re trying to look cool.”
I took his point!
Here’s a shocking picture of Victoria Beckham purportedly wearing a pair.
I’m afraid I never quite made it past the short-shorts picture in the title bar. Rowr. I forgot what a looker she was back in the day.
Fortunately, I’m not going. No reconstructive surgery for me!
My wife will fit easily into that waistline - appreciate the tip. 
Interesting about the backpack. While it makes sense that tourists would use them, in America a backpack usually means either that the wearer is a student, or that they need to have their computer with them, expecting to use it away from home. Which it is likely to be depends on the person’s age, and perhaps gender as well. Among male university students backpacks are universal; many women students use them too.
Don’t students in the UK do this? Or the kind of people who drop into a Starbuck’s and spend an hour or two working on their computers?
Acknowledged - I purposely phrased my post so as to discuss tourists of unspecified nationality in an unspecified land foreign to them.