We flew JER-LGW-LAS on the Spring Bank Holiday. Booked WTP and had been checking avidly for UuA daily from about a month out. No joy, but when we went to OLCI, despite originating from JER, we were OpUp to club (we're both SCH)

Unfortunately on the return journey, there was again no reward availability, no no OpUp and despite enquiring at check-in, we were told it was a full flight. This was confirmed when the footrest on my husbands WTP seat was broken and there was nowhere to move him (he's happy with 12k Avios though). We flew back on a Thurday night flight.

So the above suggests that on some flights there may be the opportunity get get an upgrade, paid or otherwise, but you are just as likely to be on a fully loaded flight