I have a L39A1, a British No4 Mk II converted to 308. Also have one of those Ishapore Lee Enfields in 308.
The Lee Enfield action is at its best in 303 Brit. The feed, extraction, and ejection is very positive and reliable in 303 Brit.
I have to download my target loads in the L39A1, that is a 168 SMK at 2400 fps. My normal load of a 168 SMK at 2550-2600 fps will spring the bolt. And it does not feed reliablity, even with the very expensive stirling 308 magazine.
While the Isaphore was made in 308, the feed is not as reliable as my 303 rifles. I have had bolt overrides and cartridges have jumbled when stripper clip fed. The workmanship of the Isaphore is inferior to the average British made 303. The accuracy is adequate, something like 4 MOA at 100 yards, for a service rifle.
The most fun 303 is the No 5 Jungle Carbine. That is one of the few military surplus rifles that makes a decent hunting rifle, without any modification.
The 303 cartridge, pushing a 174 bullet at 2500 fps is plenty adequate, millions of unhappy Germans can attest to that, vastly more powerful than the .223, almost as good as a fully loaded 308. And works better in the Lee Enfield.
Why change?