"We are going back to London next year."
My understanding is that 会 - as far as we've learned to use up to this point, it means to be able to/can do something. Native speakers please correct me for other uses that might make more sense here.
So your sentence could be read as, "We will be able to go back/we can go back to London next year". 我们明年会回伦敦.
Which has a slight different meaning compared to "We are going back to London next year." 我们明年回伦敦