Not worth it, keep your SMLE as is (especially if it has full wood and the nosecap). I am in the process of restoring a no. 3 and rebarreling it would just not be kosher in my opinion, especially not for the cost. The big problem with your conversion is that the .303 uses a rimmed cartridge, whereas the .308 does not (it does not stick out basically). That would require a different bolt head and extractor as well, and by the time thats all said and done, new magazine, and the most important thing with a rifle designed to shoot rimmed ammunition comes into play...headspace. Sorry to say, but it would just be too expensive to do this conversion, either get into reloading and make them yourself, or you will have to pay the price. To find THESE parts, and by the time you did the conversion, you could buy a used modern forward locking bolt rifle in .308.
I will say this though, if you really want the .308, and like the general Enfield action and milsurp appeal, the brits made a small number of No 4's in 7.62, but what you would probably get your hands on is an Ishapore factory 2A1, a factory 7.62, lots of them made, just not nearly as much as the No. 1 Mk.III's or No. 4's.
Basically, the jist of this is, enjoy your .303 for what it is. If it has full wood and you dont necessarely want it because of the cost of ammo, sell it to someone like me who will restore and make sure it survives for another 100 years. Otherwise, have fun with the rimmed ammo, its a bit more expensive, but its a fun round anyhow.
Hope this has answered a few questions for you.
