The rules of the companion voucher mean that you must fly with BA leaving from the UK. BA don't fly to Honolulu, but seeing as you are stopping off in Vegas on the way home, you could fly MAN-LHR-LAS and then buy/use Avios for your onward flight from Las Vegas to Hawaii which looks around £400/person in economy with Hawaiian Airlines - they will be a OneWorld partner if/when their purchase from Alaskan Airlines goes through so if you have enough additional points you might be able to redeem.

Club World to Vegas is 90k off-peak and 100k peak so you should make it. Use seatspy.com for reward availability - currently there is good availability April-June and December-February. The MAN-LHR leg is "free" (just taxes) and there are flights 7 times a day so just search LHR-LAS to find the required reward availability. Availability opens 355 days in advance.

You may be able to save money on the Mainland-HNL route if you add a flight (e.g. do MAN-LHR-SFO SFO-HNL-LAS-SFO SFO-LHR-MAN) but it's a lot more work to find it and I don't think you would necessarily save much to compensate for the extra flights.