Is it a sign of being gay if a male wears a white trousers in the UK?

Whilst Google has a few references on the topic (most of which date back a couple years ago), some of which do mention some sort of implicit association between white trousers and being gay, I think this belief is no more than an urban legend. I lived in London for a long time, I have several gay friends, and I have never heard them mention this, as well as I have never seen them wear white trousers, or any other item of clothing or accessory, to demonstrate their sexual orientation.

Setting aside the meaninglessness of a statistical sample based on "look around you, how many people are doing so-and-so?", and considering the white trousers hypothesis, the fact that you spent a whole day in London without meeting anyone wearing white trousers either suggests that all the people you came across are heterosexual, or that there might be no such thing as a specific piece of clothing functioning as a marker for sexual orientation. Considering the size and presence of the London gay community, I would put my money on the latter.

Finally, there are several reasons you might have seen nobody wearing white trousers. The first that comes to mind is that because people who own such garments tend to use them in hot seasons. Moreover white trousers often go in and out of fashion cyclically, and you might have had this experience on the wrong year for white trousers fashion-wise. Last but not least there's the good old "being in the wrong place, at the wrong time". Coincidentally you did not meet a single person wearing white trousers, and that's it.

There are no standard signifiers of 'being gay' in London. There are many subtle cues that people may or may not provide you with if they want you to realise they are gay. London is a very modern metropolis and even making the assumption that a man wearing eye shadow (for example) is gay might be completely wrong.

The reason you looked around and found no-one wearing white trousers (even though it was summer) is because white trousers are simply unfashionable in London. Beige or white slacks are a very USAian piece of attire. They can also present a class signifier, white trousers are quite posh.

This is stereotypical posh English summer wear. Notice the white trousers. Also notice the age of the photo...

Your female friend was simply teasing your group. This is a very common thing for people to do.

May be one of those random urban legends that goes around. For the easiest evidence, James Bond - the quintessential British heterosexual hero - wears white trousers on multiple occasions, including in Quantum of Solace.

Telling someone to look around for something and then based on not seeing it, is an example of a logical fallacy.

For example, Ben is a fish, fish can swim, therefore anyone named Ben is a fish.

They're more of a summer clothing item, were fairly common when I worked there, and unless many, many people that I knew were hiding something, the claim is incorrect.

Wearing certain colours of clothing is mostly more a fashion thing than a 'gay' thing.
Some years some colours are just not in fashion and unless in fashion, white trousers are hardly worn due to the impractial nature of the colour for trousers.

And more about white trousers: Those are standard uniform for UK rowing umpires, along with a black or dark blue blazer and white shirt, often covered or replaced by rain gear.

My boyfriend was a London resident and did umpire many a Thames based regatta in the London area back in 2014, as well as in the 20 years before and the time since, and always wore his white trousers.

That did include going to and from the event by bicycle when the weather was good enough.

I have been to some of the events and all umpires wore white trousers when the weather was nice enough.
While I can not be sure about the sexual orientation of all, most I met were married, (in a one man, one woman situation) or were having less formal partners of the other gender.

Wearing white trousers is not the sign of having a "gay" sexual orientation.

What may be true is that wearing white trousers may be seen as a sign of "weakness" or "lack of manliness."

Even if that accurately reflect's someone's personality (and often it does not), a man's physical or psychological "weakness" does not translate into "preferring to sleep with other men." It's just that people wrongly believe this to be the case.

True homosexuality is determined by behavior (known within the community), not by dress. Absent behavioral indicators, "clothes do not make the man" in this case.