'cracked the password'
So, that means either:
- The files were encrypted, and GCHQ had to either brute force the password or apply some secret super-math or backdoor technique. Brute forcing is quite possible, if it was a weakish password.
- The files were just stored somewhere overseas, and it was easier to call in GCHQ than to go through the paperwork of an international warrant.
- The files were stored somewhere, a simple warrant would probably have sufficed, but someone on the political side wanted to give GCHQ a chance to share in the glory and help improve their reputation by helping convict not just a real criminal, but a pedophile - the most loathed and hated of all criminals.