Blanket Surveillance Goal
tl;dr Phone pwning the best current option but it moves where the backdoor is, not that it matters as the government and GHCQ are after blank surveillance.
The UK government and spy agencies want automated blanket surveillance of all UK individuals as their end goal, not just potential terrorists that are used as their reason to sway public opinion. They will ignore any advice such as from this ex-agency guy as it does not fit their end goal.
We know all the recent terrorists were reported to police and authorities about their radicalisation and worry that they might do something. Encryption is not the problem for failing to act on those public tipsters.
The big agencies have tried to get backdoors in US and other countries products using 'do it for your country' and when that has failed they have tried huge bribes, along with hacking the companies and trying to insert their own bad code to take advantage off. It's just harder for them to commit changes unnoticed now. Their ideal situation is implementing an implementation or mathematical backdoor that allows decryption easily,quickly and with minimal cpu cost but would be next to impossible to find by security researchers. I have no doubt that a few of these are in play anyway.
The gentleman's point about going for the end phone has always been the best option over blanket surveillance. Targeted rather than being lost among all the information in a needle in haystack scenario. This still relies on their being vulnerabilities and backdoors in phones and the telco system that are not patched so they can keep using the vulnerability. So ultimately a backdoor anyway and no doubt these agencies are pushing for weaknesses in newer implementations of LTE5 so they can keep using the same cell network protocol tricks they use now.
So the endpoints are better than backdoors in encryption, but you have just moved the place where the backdoor is. I would like secure encrypted chat AND a secure smart phone. The phone hardware has it's own backdoors/exploits (hi US company Qualcomm) that have issues before we even get to the buggy software on the phone, that doesn't need NSA/GCHQ weakening as for example the Android Media Framework will keep giving fresh exploits in the way we have seen for flash on desktops.
Sure target terrorists but somehow find a way to do it where I can keep a secure phone too.