Hi
Just consider you are not just paying for the cost of materials and time, you are paying for someones knowledge, the time and cost spent at college learning, the constant cost of updating this knowledge to the latest regulations including mandatory ownership of current regulation guidance notes, the purchase of test equipment,calibration of test equipment purchase of torque drivers to ensure the cheap consumer unit you mention is fitted according to manufacturer instructions, public liabilities insurance, cost of suitable tools, van insurance, van road tax, diesel for van to get to your hose, the time spent doing the job, the time afterwards filling in paperwork, tax on the money you actually pay the person, notifying the local authority and most importantly your peace of mind in knowing the job is done properly and will not cause a fire, that the installation is to regulations and will not electrocute you or your family.
When you take all the overheads and tax away it is about right, some customers seem to think that as self employed you don't pay any tax and all of that money goes straight in your pocket, it doesn't! £450 seems about right to me.