If you think it will really be worth it, I will get a professional to level it with SLC.Sorry to say, but whether or not you like it, the best way to level this now is to use an SLC, and if needs be sand that afterwards. The best way to find an appropriate underlay is always to talk to the manufacturer of your flooring. They will tell you which products are suitable and compatible.
It is always better to do a job right, and put the floor in properly, in the first place, though
I'm just making sure I don't throw good money after bad... I'm guessing it will be quite expensive to level, given it's chipboard and in the loft, so sealing will have to be very thorough.