Top Tips.

Just search for an Avios redemption LGW-MCO in J without the voucher.
Check the dates, then ring up to book GLA-LGW-MCO with your voucher, it's a J booking and you just pay W Avios.
The reason is that searching for region-London-Somewhere with a voucher usually just doesn't work with the Barclays voucher.
They should waive the £12 phone fee as the booking cannot be done online.

You can almost always get CE redemption to add on to a long haul booking when done over the phone when done well in advance.

ABZ will force you to go ABZ-LHR-LGW-MCO which you may not want. I don't think you can use Loganair on a voucher.

You *could* split the bookings as you have only one voucher and three people can't use it, but 1 or 2 can. Not sure if this is needed.