Hi Eastbourneguy ,
I am not sure what if any penalty can be applied to you upon your return journey if you simply do not continue on to Amsterdam, however, I am certain BA will not check your bags only to LHR if your layover is within the same day. I have tried this often when I come back from MCO to AMS via Gatwick as I take a long layover (8 hours) to be able to go into town. I try to claim my luggage as 1) I like to do some shopping in town and want to put that in my luggage, and 2) I prefer arriving in AMS with the green sided EU baggage tags as AMS customs agents do enjoy picking on non-green sided tags for inspection. I fly this route every 7 to 8 weeks, and have never been able to claim my bags/check my bags only to LGW. I Am not sure if to LHR it would be any different, but it stands to reason BA does this to ensure people are not able to do exactly what you would like to do.