I entirely appreciate this way of thinking, particularly when you're newly self employed. You tend to compare and project your own potential salary against those who are in full time employment in a similar field. However, your fees as a self employed worker justifiably appear to be so much greater than the salary of an employed worker. You are looking after your own future after all. Expenditure is on your shoulders. You have none of the benefits that may be afforded to an equivalent employed person (pension plans etc.). You have to look after your own workflow. You can't assume that if you charge £9000 for this month-long contract that you are going to get 12 such contracts in a row every year and then you're on £108,000 a year. If you unfortunately get no work for the month following then you've instantly halved your income. What would happen to your income at this point in time 6 years ago when the recession really took hold? Imagine what a company would charge per hour for your services if you were in their employment. That figure wouldn't look remotely like your salary per hour at that company.
I do understand. It's not easy and I took the same approach when I was newly self employed but with hindsight I wish I'd been at bit bolder because working for cheap didn't build me the type of client that I wanted to work for and if I'd have regularly pitched higher then I might now have a few more savings in the bank (maybe!).
£300 per visual doesn't sound like enough basically. There's always going to be unforeseen time that goes into each render let alone the sheer admin of processing each one. Stick with it if you feel that you're going to lose your first big contract and that there'd be no room for negotiation but my advice would be to try and break out of that mindset sooner rather than later. As you say, hopefully as you gather your first few clients it allows you the breathing space to do that. The point I was trying to make was more about your salary comparison. Don't be afraid of the fact that you look as though you're earning much more. Businesses need to.