This is a Real national security problem. Think hard, Brits!
I think the real long term point of consideration is whether this is a national security issue, not what America might or might not do. I agree that "national security" has been stretched badly over the years, mostly because it has been left as an "out" in both human rights and trade related treaties. That should not eliminate the idea that there is such a thing as an honest-to-God national security threat. In fact, the whole stretching has backfired somewhat on the abusers, since in stretching the term so widely now even for national security matters we must have standards - standards that might just stop you from acting promptly against a genuine threat!
National security is by nature preventive. If danger has become actual, you are no longer so much protecting national security as restoring, repairing or recreating it. The real measure is, Can you delay the decision to preempt and still take the decision later, and how much it will cost to repair the damage if you bet wrong.
So you should think what happens IF there is a deliberate leak, or a bunch of "accidental" leaks that are creating a large loss of data. Does Britain really have the capability to guarantee timely detection, does Britain really have the gumption to flatly tell Huawei (and thus China) it is backing out, and can Britain really change its net to alternate suppliers at that point with sufficient speed and at a bearable cost.
If not, then this is a genuine national security choice and should be played on the side of caution.