Stunmilie,

How cold does it typically get in your garden during the winter? From what I understand, southern England has a surprisingly mild climate due to the Gulf Stream influence; and, if you're in London, you will probably benefit further from the urban "heat-island" effect.

I've read that mature macadamia nut trees are hardy down to 24 degrees F (although young plants are more vulnerable to the cold). M. tetraphylla is thought to be somewhat hardier than M. integrifolia. So it seems possible to me that your seedlings might do just fine outside, particularly if you have a sheltered location (against a wall, etc.) for them.

Macadamia nuts are grown by hobbyists throughout much of California, and I've never heard of special rootstocks being used to confer additional hardiness. But I'm no expert on this. I am, however, quite dubious about whether macadamias would be graft-compatible with oaks or chestnuts.

Lots of people seem to think that macadamias are much more tropical than they actually are. In the United States people associate them with Hawaii, but they really seem quite happy in warm temperate/Mediterranean-type climates.

Finally, I have to say that I doubt you could ever harvest nuts from your plants, even if they thrived vegetatively. I don't think that you would get the heat-units necessary for the plants to set a crop. (From what I read, this is a problem with a number of fruiting plants in the U.K., e.g. pomegranates and loquats: the plant may grow but never set fruit.) However, macadamias are gorgeous trees and are certainly worth growing as pure ornamentals.