A London DLR Zone 3 to Hastings route HS1 Anytime Day Single is (assuming you are travelling via Stratford Int) £46.10. I have it in my mind they do offer off peak tickets from London City to HS1 stations but if not to Hastings its £1.50 contactless to Stratford International and then super off peak single +hs1 to Hastings £37.80. yes its more expensive than the Southern ticket but it has no evening peak restrictions. The problem with the Southern ticket quoted is its via Haywards Heath that means DLR to Canning Town, Jubilee to London Bridge, Thameslink to East Croydon/Gatwick/Haywards Heath then Southern to Hastings via Eastbourne. That's not a problem from Heathrow but it quite a detour from London City.