Thinking this through, I cannot see how you would miss this connection tbh

Some tips to make the transit go more smoothly:
When you arrive at KLIA2 [if you take the train], come up the escalators to the concourse, go through the turnstiles and turn left into the corridor, then right into the main mall. Keep walking through, skip the first set of escalators and keep walking to the right and forward, then up the last escalators - this brings you up directly into the departures area. Bag drop/document check counters are in Row V [check the screens to be sure]

I'd recommend paying for a Hotseat if you haven't already done so [under Manage My Booking on the AirAsia website] - cost is bugger all to skip the boarding queue and grab a decent seat in the front rows. Pre-booking a meal is worthwhile too - there's some excellent meal choices for AirAsia and they are cheaper to pre book.

Finally, KLIA2 is a huge terminal with long walking distances. I like it for a bit of exercise, but if you're feeling stuffed after coming in from the UK - then flag one of the complimentary golf carts just as you come through from immigration. Clearly signposted, and just wait a few minutes if there isn't one there as they come by regularly.