A really wonderful thread- thanks so much for all of the helpful info.

I have successfully booked 2 of these already but for some reason
it seems more difficult this time.

The dates I would like re coming up soon so I tried
a few "practice bookings"

My questions:

1. I understood from the Wiki that we could
snag our outbound and then call in for our inbound later-
but the mobile app at least, will only find seats when I book as roundtrip If I select my certificate and choose 1 way I get a red alert saying they don't fly that route (I've tried several different cities to London and get the same message). Any idea why? Will I be able to first book just my outbound?

2. I realized today that although I believed I had sufficient Avios in our account for RT First Class West Coast to LHR that I had priced at off peak. Just in case I want to transfer more in- can you confirm how many more I would need?

Thank you !!!