It's not a weight/economy/emissions issue, it's just something the marketing guys at Toyota UK can up with as not wanted in the UK. The 4.3 Rav (not sure about the face lifted 4.3.5) IS available with a door mounted full size spare wheel . . . in Europe and Ireland . . .As these countries also have EuroCap 5/6 emissions test, it can't be argued that we can't have it in the UK to reduce the emissions either.
There have been plenty of other people who used to frequent this forum who have tilted at that particular windmill for some time, with no result (well apart from the space saver in the 4.4 which we weren't going to get originally).
Personally I'm looking out for a head on impact Astral Black Rav for breaking in Ireland . . . I'll have the Rear Door/ Bumper assembly like a shot. . . . JollyFriar (another lost soul . . . this forum used to have so many helpful members, it's a ghost town now) did just that, and has documented the process . . . but it isn't cheap