Re: This is a Real national security problem. Think hard, Brits!
"And the US suppliers like Cisco and Juniper will be happy to hand over their source code?"
Slight straw man argument in that you answered the question based on what you wanted the result to be rather than fact - I believe the US manufacturers mentioned have already handed over source code in the past for review and will do it on request for current source code.
If you had mentioned US agencies working within these companies to introduce bugs (i.e. weakening encryption strengths etc) then you may have a point. Harder to get US agency staff into Huawei jobs...