I have 2 links for ACARS sites that show reg numbers for flights over the UK (not just LHR bound planes) and an Australia flights. They are a little tricky to navigate, but very resourceful. PM me if you would like these links.

Also, this is a great site for all LHR arrivals/departures:

http://www.lhr-lgw.co.uk/

I do admit I keep a flight log with reg numbers. My wife thinks I'm a nerd, but I think it's way cool when I step on a plane that I was on 2 weeks ago and sit in the same seat.

-Ualisthebst