When I cancelled, I think the recordings became unwatchable some 8+ hours before the calendar end date. Sky presumably put in a 'cancel' command which gets transmitted again and again during the course of some hours / days (along with 'enable' commands for boxes rented by other people).
Just get everything watched one last time before the deadline... though you may be able to get Sky to issue a viewing card which makes it work for all the free to air channels (like a FreeSat box). I cannot remember if you'd get the high definition video services of FreeSat working from a Sky HD box though.
NB There may be a fee (but hopefully they just downgrade your card for free) - it's cheaper than paying 50-250 quid for a FreeSat box though and means you have a reasonable selection of channels.
I took out the hard disk and put it into a CCTV system - the Sky data is encrypted (probably tied to the viewing card) and completely worthless afterwards.
You can sometimes see used boxes for sale at 25 quid (500 GB box) which is a low enough price for a 500 GB drive, when added to a 50 quid CCTV system which has cameras and control box but no hard disk installed... Just as one way to protect your home and save on cost of CCTV.