Unfortunately it will be even harder this way. You might get 5 premium seats from UK to USA mainland if you split up and 3 go via LAX and 2 via SFO (for example) but you will get stuck once you get to the west coast. You would have to complete the journey in Y to have any chance of getting even 2 or 3 seats on the same flight. Hawaii F redemptions are just extremely difficult to bag for 1 person, impossible for 5 people I would think. Revenue fares to Hawaii are pretty reasonable given the distance and very good for mileage earning. Perhaps pay this time and sit in the back and use the extra miles you will earn for something else another time?