I have not tried it but I think Ilford DD-X may be good for pushing HP5+. In fact, in the data sheet of developing times attached to the bottle, the lowest EI for HP5+ is 500 - no time given for 400 which is it's stated film speed.
Are you having difficulty getting any Ilford product? I know your favourite (and mine) developer was Ilfosol and that we are waiting for the mythical Ilfosol 3 to appear. After trying LC-29 and then DD-X, DD-X has become my (new) favourite developer. The only way I can quantify is that my negatives just look right (to me) when I use DD-X.