Not only feasible, but many people do longer commutes. We used to live in Eltham and my Dad drove up to Mayfair each day. Used to take 1 1/2 hours +. We then moved to a village near Rugby, so he drove 10 mins to the station, just over an hour on the train, then a couple of stops on the tube. It only took him about 5 minutes longer. Major problems were i) it cost an arm and a leg, ii) he got a seat about 1 time in 3 if he was lucky.
Brighton - London is definately doable, but your 'quality' of travel and life may suffer.