There's allot of choices.
(From Stephen Redgwell's site www3 .sympatico.ca/shooters/303Page.htm)
Meredith 303/22
22/303 Varmint-R (22 Varmint-R)
22 Wasp
22/303 Sprinter
22/303 (22 Rocket - 22-4000)
22/303 Epps --- almost doing 4000 fps
22/303 British Improved 40
220/303 British
243/303 (6mm/303 British)
6mm/303 Epps
25/303 British
25/303 Epps
6.5/303 Epps
270/303
270/303 Epps
7mm/303 British
7mm/303 Epps (7mm/303 British Improved)
7.7x54mm Rimmed
303 ICL Improved
303 Epps Improved (303 British Improved)
8mm/303 Epps
338/303 Epps
338 JDJ
35/303 Epps
375/303 Epps
**there's a word of caution about the 303 Epps, There are different loads for each type of Lee Enfield action. In fact the No1 it would be a waste of time doing a conversion, The 303 Epps is best done on a P-14, I would also think the same for some of these calibers as well. Don't forget to see if you could do a conversion on a Ross MkIII sporter, That receiver can take any caliber you want.
Pete