If you don't have enough miles in your MileagePlus account to ticket an award there will be an option to put the award on hold.

To be honest, I would make no promises that the MCO-DUB-LGW will ticket — I figure 5% chance UA claims that their partner EI didn't really have the space and leaves you holding a pile of miles. I think if that did happen, you'd be able to escalate to a supervisor who might be able to force open UA space on something like MCO-EWR-LHR and book a similar trip at the originally agreed-upon mileage price.