Guess you'd need to caveat 'lowland site' since no doubt there are upland cairns in Scotland which would take a day's walking to get to!
I guess Rubh an Dunan on Skye is about as remote as I can handle... near on an 8 mile round walk from the nearest road:
http://www.themodernantiquarian.com/site/1700/rubh_an_dunain.html