The British government wants to build backdoors into standard encryption libraries. This means the technology that protects your financial transactions could have a backdoor. These same backdoors were just exploited in the WannaCry outbreak that just took down the NHS. Who created that backdoor? The NSA, whose incompetence allowed this "tool" to fall into the hands of a Russian government-affiliated hacking group.
You don't do anything illegal? Great! You do have a bank account, right? You do have private conversations with your significant other, right? Would you want either to be open for potentially anyone to see? Probably not. Resist government's demands to force Internet companies to open backdoors for them. Nothing good will come of it.
Backdoors are bad. There is no way to guarantee access to one group of people to a backdoor. People are fallible. Strong math