we shouldn't trust the US
We don't (always), hence there is such a classification as UK EYES ONLY!
the Americans shouldn't trust us either
Very sensibly, the Americans don't trust anyone when it comes to defending their sovereign airspace - they do it all themselves! As should we defend ours. Collective defence arrangements such as NATO play a major role in international relations, and have obviously been vital over the years in stabilising global affairs, but when it comes down to it there is only one nation with the UK's interests at heart - the UK. Just because piddly nations like Luxembourg and Lithuania rely on NATO to defend their airspace doesn't mean we should aspire to the same.