I like the 303 also.
But if I was building rifle in that caliber I would go with the Enfield.
The 303 is rimmed cartridge and the bolt on an Enfield is already opened up for it.(unless you start with a 30-06, 308 caliber Enfields)
Also the headspace is easy to set, just change the front part of the bolt to the correct thickness. Also you already have a magazine that you won' t have to modify.
If you start with the right Enfield they look pretty good.