Does anyone know where I can get access to a rooftop in London for photography purposes?

You could speak to the facilities manager of a building and inquire as to whether you may be permitted on its roof, but to be honest, your request will likely be denied due to insurance purposes- if you were to fall, they could be held liable. Professional photographers gain access to rooftops all the time because they carry their own liability insurance, but in some instances they may just prohibit photography on their premises as a matter of protocol or policy. So beyond suggesting that you use more "creative" means of gaining access to a rooftop, there's not much advice I can give, sorry.