This is commonly asked in relation to "Hidden city ticketing".

It is down to the checkin agent and airline policy whether they will allow you to short check your bags. If you have a short connection at LHR they may be reluctant to short check them.

However, your bags will not travel on the AA flight if you do not board. It just make take some time for you to be reunited with your bags if they are not short checked.

You may want to look in to changing your existing booking to include a stopover, rather than pay for a potentially quite expensive one way ticket.