Not correct.
You can use any cast bullet load with a jacketed bullet of the same weight. The cast bullet loads act at pressures well below the strength of any action, including a Lee Enfield, so any increase due to it being a jacketed bullet remains safe. Contrary to common belief, the increase is not very large.
The first question should be: is it to be shot in a Lee Enfield? P14? Ruger No. 1?
Furthermore, with the dearth of 303 Brit loads that are published, you can use 308 Win cast loads (even jacketed loads in a strong action lke the P14 or Ruger No. 1), as their case capacities and bore diameters are very close. In fact the 303 has slightly more case capacity and a slightly larger bore diameter and that lowers pressure below what that load would produice in a 308 Win.
Here are suggested loads for the 303 Brit with a 150 gr jacketed bullet with IMR-4227 (from the Lyman Cast Bullet handbook 4th edition pg 149):
1. 18.7 grs - 1710 fps - 14.2K psi; and
2. 25.5 grs - 2177 fps - 25.1K psi.
Or anything in between.
If anyone thinks I'm mistaken, please explain how, and I will consdier your points carefully and respond.
Not sure why I got the "not correct" quote. I made no claims other than my understanding from the reading I have done on reloading the .303 , mostly on Milsurps.com, and the list of powders I have gathered for 150 and 180gr jacketted bullets. IMR 4227 is not on that list. As a novice reloader I make no claims to be proven incorrect or correct..... but all the reading I have done only mentions the IMR 4227 for cast loads..... that's not me saying it can only be used for cast loads. Big difference there because I in truth have no idea LOL
What I can say is that as a novice reloader , in my researching powders in these scarce times I realized that there are a crap load of powders that can be used with .303brit Jacketted bullets
Alliant : reloader 10x, and RL15
Accurate : 2230,2460,2495,2520,2700,4064,4350
Hodgdon : BLc-2, CFE223, H335, H414, H4350, H4895, Hybrid 100V, Varget
IMR : 8208XBR, 3031,4064,4166,4320,4350,4451,4895
Vhitavouri : N135, N140, N540,N160
Winchester : 760, Staball 6.5
I am sure that list is not complete but just shows the OP that there are other options that might be more suitable for the actual bullet he is reloading for.