If you buy Huawei kit you can either cherry pick bits and pieces that you want and then combine with kit from other manufacturers, or you can buy a complete turn key system from them.
The UK's mobile operators want to pick the best technology from a variety of suppliers and combine it into a system using their own engineering staff, or to contract companies in the UK (or elsewhere in Europe) to do it. There's already loads of Huawei kit in the UK's existing 4G networks because of this and it isn't going away regardless of 5G.
In many third world countries however the mobile operators simply buy a complete turn key system from a single supplier, and unlike many other companies Huawei is capable of doing this. It's not what UK companies want to do however.
Security comes down to system design, not the individual bits and pieces. When you buy a label there are no guarantees of security coming with it.
And in the end, the alternatives may be headquartered in Europe, but the actual kit is made is made in places like India, who have their own ambitions for being a world power.
Oh, and a lot of Huawei's software is written in India. The world is a much more complicated place than it was in the 19th century, which is where a lot of people who put national labels on multi-national businesses seem to have their minds stuck in.