I live in Helston and locally there are bits of riding to be had, cheeky coastal paths from the Lizard to Helston (Loe bar), properly rideable from about Mullion Cove down towards Helston. From Mullion to the Lizard the coast is epic with some good bits but a lot of bike on the shoulder too. Loe bar aint bad for a blast either and there are bits to be had around the Gweek area too but it’s all a touch bitty tbh.
Further afield Carn Brae (nr Redruth) is pretty good for technical rocky riding and Poldice valley offers quite a variety of trails for more dh/jumpy orientated riding.
Check out Cornwall freeriders on facebook (name a bit misleading as we do cover all spectrums of riding). There are a few maps on there i think for local trails.