I am an atheist, from a very religious family, and have taught RE in a rural C of E school and in an in-town non-denominational school.
IME the balance of the RE curriculum tends to play out over time - so there is not an equal balance of all faiths in every term, or every lesson, or even every year, but over the primary school years there IS a good balance of faiths.
There is, in all RE curricuula that I have used, more apparently 'Christianity based' topics than any of the other faiths, but many of those 'Christianity based' topics are of a 'general moral / ethical' type. For example, the one about 'looking after creation' is is, in our curriculum, started off with a look at the JudeoChristian Creation story, a comparison with the Muslim version, a general look at what humans have done to the world, and then a look at responses from people of a variety of faiths and none, looking at Eco and world problems and those who work to solve them (comparing, for example, the work of Christian Aid with similar but atheist charities, and discussing any differences in approach we find).
If you as the school for a full statement of the RE curriculum, perhaps even at quite a detailed level, it may give you a better idea of whether you feel there is 'balance' or not.
As an atheist, I use the formula 'Christians believe...', as I also use the formula 'Jews believe' etc, but I also discuss how, as a country with a long Christian history, many 'Christian' ideas / festivals / values / approaches have become part of our national life and 'how we do things' (e.g. holiday dates) even though not everyone would say that they are a believing Christian.
I enjoy teaching RE, and my pupils seem to enjoy leaning about it. I have never had a child opt out of RE, though I have had pupils of other faiths opt out of assembly.