Again, I think that the human player would mostly feel dumb if the scenario is handing him free units to do a scripted invasion. For the AI on the other hand, sure...
But, you won't be able to spawn those units of yours if there already are cities in Britain. Because of the culture will prevent those units from spawning. I'm not really seeing how it would work within the bounds of what you can achieve working with the Barbs.py file. (Almost anything is possible if you can do your own code altogether, though.)
A more reliable way would be to convert those cities by doing a second spawn, but that would be even more complicated to code. I'm willing to give it a go, nonetheless...
Also, the reason the AI isn't attacking Independents is that they aren't at war. All players are always at war with the Barbarians though. But, its possible to make Civs declare war on each other with Python. (This is in fact what happens when Civs spawn and they end up being at war with some of their neighbors from the get go.)
Others might wanna weigh in with their opinions here also, even if you first and foremost are working on this to suit your own play. Also, be mindful about never letting your mod become finished (there's always more to add, if you don't set goals that you'll eventually be able to reach). All the play testing involved with modding will make the finished mod feel like you've already done it to death. This is mostly how I feel about my own work, but then again I'm modding for moddings own sake.
If I end up actually playing the finished work someday, all the better. If someone else does, that's also good.
