Depending on how long you have it can take a while to credit from the Avios store. Although you have a "few months" so should be fine.

If you ever get caught short with less time then you just book a short EU flight in Y for 4,500 Avios + tax at least a couple days out. Wait a few hours and cancel and you get ALL Avios + taxes back except for £25. So in essence it costs £25 to extend the miles 3 years which can be worth it if you have quite a few.