All Comms kit has back doors in it. Its about choosing whom you allow to put them in.
Cisco was riddled with backdoors, and if you were interesting, the five eyes would specially intercept your kit and put extra bits in before it was delivered to you.
The issue here is geopolitical. Who has control of your friend's internet.
Now there are few ways to look at this:
1) this is the present government's master plan to use this as a bargaining chip in negotiations
2) The kit is going to be at the edge, which if the network is designed properly, will mitigate attempts to snoop or spoof
3) GCHQ et al, can't plan for shit.
4) The government and civil service can't plan for shit, don't understand how tech can influence geo politics
5) Trump is trying to get allies to buy shitty expensive kit from the USA.
Out of all of these 4 is my favourite hypothesis, but there are bound to be more factors. Yes, Huawei have sponsored a lab to prove that their kit is secure. But of course they would, they are not going to ship gimped kit to be tested.
Also, As Huawei kit already operates vast parts of openreach's network (the company that runs the vast majority of landlines and fibre) the point is kind of moot. The decision to allow this is why marconi bit the bucket, killing the last bit on comms design the UK had. (like every other pioneering electronics/computing company in the UK.)