This might be wrong in a couple of ways.
First - without knowing any details, there certainly are many good reasons why the information might not made public, most obviously because it would possibly compromise their methods of acquiring such information. There's a 100% chance that any information they divulged will be acrimoniously parsed by the Chinese to determine the means by which the US obtained the information. The information will likely betray how the Americans are thinking about the problem as well. (Reference yesterday's WaPo article on US/Germany surveillance program and how it was compromised).
Second - there's very limited upside in making the information public, are many downsides. The US, UK, and German agencies don't care about you, or I or any other privacy advocate, or really the knowledge that might arise from a number of public voices piping in. This isn't like open-source SSL software, where it makes sense from a security perspective to be open about it.
Also, I'm doubtful if the 'evidence' the US presented is purely technical in nature. My guess is that this is not a situation of "hey, look at this source code, on like 501, there you go, back door!", it's probably much more complicated. If you read the article, you'll indicate that the US has evidence that China is using its gear to surveil state actors, etc..
Revealing the 'evidence' almost surely involves sensitive information regarding state officials from other countries, insight into US spy programs etc..
If the US is legitimately trying to convince UK/Germany that Huawei gear is a legit threat, then providing them with the necessary information in confidence is obviously the rational thing to do.
The British badly need to negotiate pst-Brexit trade deals with all major powers. Pissing off the CCP before the negotiations won’t help the UK’s plans.
Like 'you know how your mobile phone system works quite well, let us help ruin that for you by messing with it politically', great.
In any case, my point is not that they aren't plausible reasons why they might not make any proof public. My point is that they aren't above using those plausible reasons to propagate disinformation. So their statements are meaningless to us (the public) because it's impossible to discern truth from lies without further information.