When I had the same cancellation from SOF-LHR, they said I could drop the first leg but only with a re-price with LHR as a starting point, which was considerably more expensive. You may be luckier.
However, they let me change start point to OTP - which then didn't meet the other TATL flight timings, so they allowed me to re-schedule all of the connecting flights from LHR. You may be less lucky.