Bearing in mind they were already free it can still be worth getting them tested. If you can get them to a tester then it shoikd cost under £30 each
Then you either have two tested cylinders or can pass them on.
Failing that drain them and remove the regulators. If you want to hang on to the regulators then take care disassembling, otherwise you can use whatever force you like
Then put the disassembled cylinders to aluminium recycling
If you are not so sure about disassembly then have a word with the guys at the local dump, they may be able to accept them without disassembly, I have seen scuba cylinders accepted at my local, and have also seen hospital oxygen cylinders go through an incinerator. However that was a specialised, sealed, computer controlled 'green' incinerator / power generator