By my calculations:-

A Gold would earn 13832 Avios on a round-trip WT+ (T class) LHR-JFK-LHR paid ticket; 17290 Avios in CW (I class); and 24206 in F (A class).

Buying the WT+ ticket and UuAing to CW would cost £1356; you'd spend 48000 on the UuA but earn 13832 back, so the net cost is £1356 + 34168 Avios.
Buying the CW ticket on its own would be £1735 and would earn 17290 Avios. So the UuA saves £379 but costs 51458 Avios net. You save 0.74p per Avios spent.

Buying the CW ticket and UuAing to F would cost £1735; you'd spend 36000 on the UuA but earn 17290 back, so the net cost is £1735 + 18710 Avios.
Buying the F ticket on its own would be £2135 and would earn 24206 Avios. So the UuA saves £400 but costs 42916 Avios net. You save 0.93p per Avios spent. (Looking at spending the further 55000 Avios on saving £300 off the F fare, you save only 0.54p per Avios spent.)

Am I making sense?

I can see theoretical fares in WT+ for LHR-JFK-LHR for something like £750 round-trip. If you can do a UuA to CW from a fare of that level, you're going to get much better value for your Avios. And all of the above ignores the TPs you'd earn on each fare, which complicate the assessment of value.