You will be aware of the risks of separate tickets, so if you are 6 months away then you may want to consider what Plan B looks like if your VS service is delayed.

If you go to the BA Forum Dashboard you will find a guide to connect at LHR and specific steps and photos to make the connections. If you have no checked luggage you can go airside, by going to Flight Connections in T3, bus from there to T5, Flight Connections and security in T5. I have done it in just over 30 minutes gate to gate, but if you assume 1 hour then normally that covers it. By all means check nearer the time for up to date information since recommendations may well change between now and then.