Could a SLR rifle bullet be used In a 303 rifle both?

No. The L1A1 Self Loading rifle in in caliber 7.62 x51 mm (7.62 NATO) fires a rimless cartridge with a .308 bullet. The .303 Enfield cartridge is a different shape, has a rim, and fires a .311 bullet. If you compare the two cartridges, totally different in shape.