Sorry for this question as I'm sure it must have been discussed to death, but I couldn't find anything from a quick search.

I'm flying long-haul into Heathrow in a couple of weeks, but now need to be in Manchester that evening. So I plan to book an LHR-MAN flight, as a separate booking.

If the inbound lands on schedule, as it has been doing for the last few weeks, there's a perfectly timed LHR-MAN flight I could catch. But I need to manage the risk the inbound is delayed.

Free same-day change on a revenue ticket would seem to be perfect for this, but it would be much cheaper for me to use Avios, and there's good availability. I understand I have no right to change my Avios flight on the day. But can I simply book two flights and throw away or refund the one I don't actually take? Or will the system see I have booked an impossible itinerary and reject/cancel one of them?

(Yes - I know there's still risk, eg in the case the inbound is massively delayed and I miss both Avios flights)