Can I use the steel wire and a spare core to earth Armoured cable?

Fox Mico Ltd could I just point out that you can certify your own work without being a member of a competent persons scheme. If you can satisfy building control that you are a competent person with the relevant qualifications that is acceptable. I am an electrician and was an apprentice for the local electricity board over forty years ago I have all the city and guilds required and more. I am NOT a member of any competent persons scheme and I certify my own work with the approval of building control.

You can Earth it from either end from the core you are using as the Earth. Or, if you prefer both ends.Dont know how you are going to do r1+r2's insulation test, Earth loop impedance test the RCD's produce a certificate and notify to building control though.

to expand on the above reply, it sounds like you are doing the work yourself, this would be illegal due to works been notifiable under building control regulations... unless you are qualified and part of a self notification scheme, which I guess you are not otherwise you wouldn't be asking this question. The amount of times I have people tell me 'I wired this' and 'I wired that' myself and it works fine... difference between electrical and many other trades is, when it's unsafe, it still works fine and it's unlikely anyone can tell it's unsafe without expensive test equipment and 4 years in an apprenticeship and a further 10+ years experience etc etc.

The simple answer is yes, as long as the steel wire armour is connected to the main earth at the source as it protects the cable so that it disconnects if the cable gets damaged.

YES, You can Earth it from either end from the core you are using as the Earth.