The UK is caught between a rock and a hard place (well worth a listen).
On the one hand side, you have a big trade partner that's made all the more important with the looming Brexit, China, and its national champion, Huawei. The latter is enormous (because of China's own market), reportedly has better and cheaper equipment than the next two big players (Ericson and Nokia), and accuses the US of creating a smokescreen for protectionism.
On the other hand side, you have an important strategic partner that is also made all the more important with the looming Brexit, the US, with its trade war with China, and its security concerns over control of NATO's core 5G network.
The evidence for security issues and potential government overreach is mixed. GCHQ got to review Huawei's source code and found no backdoors. However, they also held no punches and basically spelt out that what they saw was a mess of spaghetti. There's also concern that, while Huawei seems independent today, China could tell it to do a few things down the road -- and the Chinese government isn't one you can really tell no to.
Anyway, there was a recent leak that suggested the UK would allow Huawei to supply 5G network equipment except in core parts of the network. In doing so, May's government would make (already has made...) both sides unhappy. But then, there is no good choice here. If she sides with the US, she'll really piss off China; and vice versa.