This is a good example of the kinds of tradeoffs which must be made at the highest levels of public service.
Huawei, just like any Chinese corporation operating overseas, is an attack vector for intelligence gathering. Anyone presenting a counter-argument to this is either a shill for the Chinese government, or totally uninformed.
China has a culturally distinct attitude towards intelligence and intelligence gathering to nearly every western country. The national emphasis on the collective good blurs the line between private citizens, acting in a personal or professional capacity, and the stereotypical impression of a "spy" perpetuated in the west: on the payroll, going to their cubicle at the CIA each day. China's voracious appetite for intelligence (and, particularly in recent years, industrial espionage), means that it is impossible to distinguish between the commercial interests of a Chinese company and the Chinese state furthering its apparatus.
Remember Crypto AG? The Swiss crypto company jointly-operated by the CIA and German intelligence?[1] That's newsworthy because it's unusual: western states are typically limited to publicly lobbying their corporations for backdoor access, or working around things like end-to-end encryption (e.g. I believe PRISM used a combination of vulnerabilities to exfiltrate data from Hotmail and MSN prior to encryption taking place).
In China, we must assume that the reverse is the norm: the Chinese government does not need to lobby its companies to provide it with data, or to build-in backdoors or exploits. A Chinese corporation can be compelled to turn over everything it has, silently, and to compromise users and products to benefit the Chinese government, silently.
Crucially this is not a criticism of China. China can best be understood by Westerners as a series of tradeoffs to benefit the collective good, at the expense of personal liberty and privacy. Literally the argument you might encounter would be: "If you have nothing to hide then why do you care?"
The information gathered is not always as exciting as you might imagine. It's not just deployed into military intelligence or kompromat. It might "just" be used as a means of preserving China's status quo as a leading manufacturing hub (and, therefore, China's position as a growing economic power).
So China a) has a vast appetite for intelligence of all kinds, and b) does not draw a distinction between private citizens/corporations and state actors/corporations.
To answer your question:
Huawei has been a cornerstone of the UK's telecoms infrastructure for nearly twenty years, and in order to gain its foothold committed to allowing GCHQ full access to its codebase (HCSEC)[2]. The stipulation from Britain's intelligence community was that Huawei must not be allowed to have a monopoly position, or even a significant market share beyond a certain level.
I am not familiar with the specific technical reason that Huawei at 70% vs. Huawei at 40% of the UK's telecoms infrastructure would represent a disproportionate increase in risk, but I believe it is likely to be related to resource constraints -- fuck me guys, GCHQ is having to actively monitor and review the code deployed across a double-digit % of our telecoms infrastructure from the starting position of "this is provided by a bad actor"! -- and the doomsday scenario that Huawei's position of market dominance would drive competition down, resulting in a choice to either have e.g. 7G with Huawei, or not at all (7G is a fictitious example, but you see my point).
The UK is balancing the very real ongoing nightmare of monitoring Huawei's involvement in UK telecoms with the fact that it's a cheap, high quality supplier, and the fact that our closest allies -- the United States -- have been on a warpath over Chinese intelligence gathering since long before Obama put the kibosh on China acquiring Aixtron in Germany for national security reasons. Oh, and we want to get a trade deal out of the US in the near future.
The risk:reward for Huawei is at a point where it's no longer sustainable. Phasing its removal from our infrastructure will smooth our relationship with our closest ally, reduce our reliance on a Chinese state manufacturer, and reduce the workload on our signals analysts in GCHQ.
[1] https://www.theguardian.com/us-news/2020/feb/11/crypto-ag-ci...
[2] https://assets.publishing.service.gov.uk/government/uploads/...