Advance tickets, routed EC & Connections are available to Folkestone. (Try specifying via Thorne North or something to get a Northern connecting train to Donny, then EC from there). Use the EC website and you will get 10% off. (SET apparently do not like people using Advance tickets on their 'shiny new trains' but they just have to lump it!)
Alternatively you can use a combination of Hull Trains advance to London, and a walk-on ticket from Folkestone. However given the extortionate cost of the latter, I would advise checking for EC tickets first!
Unfortunately Hull Trains do not offer '& Connections' tickets.