Trademarks do get grouped into classes (the so called "Nice Classification", like the French city [0]). The classes are very detailed, and the first 34 refer to "Goods", the rest to "Services".
Specifically class 9 refers to (among many other things), "computers, computer software", class 42 to "design and development of computer hardware and software".
However those are mostly just to simplify searching for and dealing with the huge amount of trademarks. Each trademark comes with its own list of goods and services it's supposed to cover. Their trademark in the UK [1] covers "computer software, software and apparatus for the extraction of business information and knowledge".
They also appear to have registered a trademark this september [2] with a much broader scope, so after Meta started using threads. I'm not familiar with UK law and how it relates to trademarks in this case, however.
[0]: https://en.wikipedia.org/wiki/International_(Nice)_Classific... [1]: https://trademarks.ipo.gov.uk/ipo-tmcase/page/Results/1/UK00... [2]: https://trademarks.ipo.gov.uk/ipo-tmcase/page/Results/1/UK00...
And it’s not without precedent to secure an injunction against as US tech company launching a new service - gmail was known as googlemail in the UK and some European countries for several years owing to a trademark dispute (https://www.theguardian.com/media/pda/2010/may/04/digital-me...)
But sadly a lot of trademark and copyright cases come down to who has the bigger pile of cash behind them.
Though that's clear case of different industries.
Threads is a much more compelling case.
(I am not your lawyer, this is not legal advice.)
I don’t think this is totally clean cut. UK trademarks are registered against specific classes and those classes are categorised in families of goods and services.
A quick search suggests that this company holds the trademark for the word ‘Threads’ for class 9 (goods) and class 42 (services).
You can read the definitions somewhere by searching on gov.uk (or somewhere like https://www.russell-cooke.co.uk/media/bmin5fo0/goods_and_ser... [pdf]).
Meta will almost certainly make the argument that they are not operating in these classes or overlapping with the categories listed for this trademark. Class 42 does not seem to apply here, and for class 9 the Thread trademark lists ‘computer software, software and apparatus for the extraction of business information and knowledge’ which may not overlap.
Meta couldn’t also trademark Threads under class 9 (can’t trademark the same word twice in the same class) but just because they can’t secure the trademark does not automatically mean they are infringing on the existing use.
They could argue that their use of Threads as a trademark falls solely within something like class 38: Telecommunications services; chat room services; portal services; e-mail services; providing user access to the Internet; radio and television broadcasting.
If they could land this then there is no claim against them - their use can coexist with trademark’s registered in different classes.
For an app which can be downloaded getting around class 9 could be difficult - so whether they could land and make this stick is far from clear, but Meta have the resources to explore this indefinitely where as the pre-existing user making the complaint may not be able to afford the legal costs to stay the course.
Trademarks are messy and subjective and there is much scope for interpretation - even a seemly clear-cut case is anything but predictable.