The bolt face isn't big enough for the rimmed 303 cartridge for one thing, that's why the SVT was converted in the past because it already has the proper sized bolt face.
Dimensions of the .303, .308/7.62x51 and 7.62x39:
.303 British
Base diameter 11.68 mm (0.460 in)
Rim diameter 13.72 mm (0.540 in)
Rim thickness 1.63 mm (0.064 in)
.308 Winchester
Base diameter 0.4709 in (11.96 mm)
Rim diameter 0.4728 in (12.01 mm)
Rim thickness 0.0539 in (1.37 mm)
7.62 NATO
Base diameter 0.470 in (11.9 mm)
Rim diameter 0.473 in (12.0 mm)
Rim thickness 0.050 in (1.3 mm)
7.62x39
Base diameter 11.35 mm (0.447 in)
Rim diameter 11.35 mm (0.447 in)
Rim thickness 1.50 mm (0.059 in)
So that is a difference of 1.71mm, or 0.855mm per side. A thicker bolt face could easily be substituted if that is too much material to remove.
There already are 10 round AIA mags for these rifles in 7.62x51 so what's the benefit?
Where? How much? Every retailer who had them at one point has been sold out for years.
The No1 has a "prescribed" mag limit of 10 rounds anyway, so anything over that is prohibited, even in the No1.
ii) is a rifle of the type commonly known as the “Lee Enfield” rifle, where the magazine is capable of containing not more than 10 cartridges of the type for which the magazine was originally designed
That is not a limit, that is a specific exemption for the ten round magazine because of the Charlton automatic rifle, because C-68 was written by idiots. The twenty round magazine is not prohibited. If it were put into production, it would be legal to own and use without blocking to ten rounds.