Working for RM will always give you a headache when needing time off. Even more so now every office seems to be understaffed. It goes with the territory unfortunately!
Your best bet is to pay/book the dream holiday now (if you want to risk losing your money?) Then when the official time for booking up holidays at work arrives, try and book as many weeks as you can then to fit in with your trip. Then try and convince (or bribe) people with any other weeks off you need to swap with you. Alternatively speak to union and see if they can intervene in anyway although I doubt there is much they can do?
Good luck!