If the scope of the project is huge then don't go for fixed-bid quote, go for a T&M quote. Also, when freelancing you need to consider other costs (e.g. insurance, medical, pension, electricity, computers, internet connection, furniture, taxes etc) that the employer will not be paying but you will have to pay. So factor that in to your quote. The London contract rates is somewhere around £65 to £200 per hour depending on the project and your skill-sets.