The Guardian compares the aerobic and cardiovascular fitness benefits of cross country ski-ing and nordic walking - read the brief article.  Both rate highly and given the similarity of roller ski-ing to cross country ski-ing, you would expect similar ratings for this great activity which you can practice much more easily in the UK.  In London you can learn via The Roller Ski Co who run sessions throughout the year, whilst London Region Nordic Ski Club runs occasional courses as well as races.