Cables within cables correct, I've found bundles of 3 cat5e, I've got two working cat6 run at the moment.
Purpose is to run my networking kit upstairs, ubiq firewall, switches etc. with a couple of storage servers, initial connection coming from the BT/Openreach OTP.
It's not a massive run so I am tempted with your suggestion of fibre, not done anything with fibre outside of traditional enterprise storage though.
Edit - Sometimes you just need to say things in order for you to get them straight in your head. The ubiq firewall I have has only one 2.5Gb wan port, so the cat6 will do for now. Everything after that will be a 1-10gb depending on device so all good.