Yeah, you can always go crazy with (off site) copies. There’s a DigiPres software system literally called LOCKSS (Lots Of Copies Keep Stuff Safe).
The German Federal Office for Information Security recommends a distance of at least 200km between (professional) sites that keep georedundant copies of the same data/service, so depending on your upload capacity and your familiarity with encryption (ALWAYS backup your keys!), some cloud storage provider might even be a viable option to create a second site.
Spare drives do absolutely work as well, but remember that, depending on the distance, data there will get more or less outdated and you might not remember to refresh the hardware in a timely manner.
A safe deposit box is something that I hadn’t considered for my personal preservation needs yet, but sounds like a good idea as well.
Whatever you use, also remember to read back data from all copies regularly and recalculate checksums for fixity checks to make sure your data doesn’t get corrupted over time. Physical objects (like books) decay slowly over time, digital objects break more spontaneously and often catastrophically.
Good to hear! When you go with the National Archives UK, you can’t fail. They have some very, VERY competent people in staff over there, who are also quite active in the DigiPres community. They are also the inventors of DROID and the maintainers of the widely used PRONOM database of file formats. https://www.nationalarchives.gov.uk/PRONOM/Default.aspx Absolute heroes of Digital Preservation.