Perhaps try and think about the project in terms of days and weeks rather than hours. For example, if you think £250 is about what you'd expect for a week's work, consider whether you'll realistically be able to complete this project within a week's constant work.

Also, it depends on the expectations of the business you're designing for. In my opinion, a full branding can be done quite quickly, within a couple of days even. It's the perfectionism and client-requested amendments that can take the real time. Basically, if your client is very pedantic with a strong vision of precisely how they want the designs to look, then it can be far more time consuming than if the client is laid back and has faith in your vision.