Responsible, grown-up politics is about facing problems and finding solutions. Solutions are often messy and involve difficult compromises but that’s (political) life. Irresponsible politicians believe that difficult problems can be avoided by pretending they don’t exist.
It is entirely correct to say that the UK can do little about climate change on it’s own. That doesn’t mean that this serious issue can be swept under the carpet and dismissed as someone else’s problem or, even worse, as a hoax.
The reasonable conclusion is that the only way forward is for international agreement on what every country in the world should be doing. Such agreements will not happen without leadership from respected, decent, clear-sighted countries such as the UK.
Claims that it’s too expensive and unachievable amount to saying “it’s too difficult, so let’s pretend the problem doesn’t exist”. If something has to be done, the grown up thing is to get on with it. The 2050 target for net zero is indeed slipping but that just means we need to try harder.