You'll never get the same speed twice because with the exception of the copper line going from your home or business all other parts of the infrastructure are shared with others which means traffic fluctuates based on what people are doing.
Virgin (ADSL) => Namesco => Newnet => O2 => Plusnet => Zen => Newnet => Zen => Freeola => Vivaciti (using O2 Wholesale DSL) => Xilo (C&W Wholesale) => Xilo (O2 Wholesale)
Router: Billion 7800N
Note: I don't lay turf for anyone. astro or otherwise, all views and opinions expressed are my own based on experience.