Originally Posted by
Virazuno
To answer your question - you can add a bag at any point in your journey. Typically, landside, at the check in counters. What Richmond_Surrey is meaning to say is that you will have to come landside through the UK Border to check your 'extra' bag in (the assumption here being that your additional bag will be landside, rather than airside).
I based my response on this comment in the OP so read it that they had left the bag at LHR
... I assume i'd have to go back through customs to get bag ...