It can take a few weeks (even 2-3 months IME) for an UuA to be ticketed. Once ticketed you should get an e-ticket receipt emailed to you.

It's apparently a manual process to do this when upgrading using Avios, tickets that have already been purchased. If your departure is imminent then you go to the front of the queue. I had one earlier this year. Ticket was purchased in January for a July departure and I phoned to UuA the same day for one of the sectors. The upgraded flight was finally ticketed in April.

If gets to two weeks before departure and you've not had a revised e-ticket receipt emailed to you then I'd phone them.

Another way to check if a flight has been ticketed is using the mobile app. Click "My Bookings" and then the flight(s) you've upgraded. Under the booking reference you'll see "Flight duration", "Aircraft" and "Ticket number". If the ticket number has a long number beginning 125-.......... opposite it then you know it's been ticketed. If it's blank then it's still in the queue. Your upgraded sector will still show as the upgraded class and seat map even though it hasn't been ticketed yet so don't take this as an indication that all is well.