All of the RTW products have rules designed specifically to prohibit doing what you would like to do (buy one ticket and take multiple trips). There are of course ways around this (legally) but they aren't cost effective for a few trips (one approach is to have multiple RTWs going eastbound and westbound, but it gets tricky to coordinate them). I think BA flies LHR-MRU (might be seasonal) but it really isn't practical for any of the Oneworld RTW products.