A transformer does not change frequency, only voltages and currents.
I know that UK mains voltages are "Nominally" 230V but in reality they are higher. My question to someone having access to these transformers, would be what is the actual voltage you see at the input and output at your location?
If it's more like 115-117 volts it would be perfect for most US vintage audio electronics. 110 would be perfect for prewar antique radios and appliances and usable for 50s audio gear. Assuming the power transformer is well designed it will work on 50 Hz fine, but if it is already marginal that might push it over the edge.
What do these transformers typically weigh and what is a typical price?
Also,,,any transformer can always be used "backwards", i.e. feeding it 110 volts in and getting 240 out. If the ratio is really 11/24 though, US mains at 120+ will give 261vac....too high for 240 volt stuff and WAY too high for 220 stuff. The US 60 Hz power will run cooler and better for a 50 Hz transformer unless it is a ferroresonant type, which these are not.