There are quite a few fell races these days in the Southern half of England. I think the first was Box Hill - first run in 1982, and (I believe) the brainchild of Anthony Kay. Don't think about doing it though - it sold out in 7.5 hours back on 1st Nov. Box Hill and Surrey Hills are organised by the same club - South London Orienteers. Surrey Hills Races involve navigation although the route is always the same. You can see the route here
https://www.slow.routegadget.co.uk/rg2/#25&course=1 Entries should open soon on Fabian4, but it won't sell out like Box Hill - southerners seem to find a navigational element quite off-putting!