Some clients when hiring freelancers don't understand why they charge so much. Freelancers don't actually charge that much at all. An agency usually charges double, triple and even quadruple what a freelancer would charge. This is of course because they have offices, employees, equipment...etc.
A freelancer actually has the same sort of assets and responsibilities as a business. That room you use in your home, that room where you do all your work, that is your office. That office requires electricity, heating/cooling, equipment, internet and whatever else is needed in an office. The costs to run that office should be put on the rates you charge as a freelancer. Just how agencies put it on their rates.
Charging £30-£40 an hour is nothing when compared to agencies so you have to put everything in perspective. Many freelancers think "WOW, I'm earning £40 an hour, that's a lot better than my previous job where I was paid £20 an hour", but then out of that £40 you'll probably only get £20.
You also might not charge clients for the hours that are taken up meeting them, planning, travelling to them, buying the odd meal - these costs have to come from somewhere, they come from your rate.
So next time someone questions your rate or you yourself question your rate, think about all those expenses you have to actually run your freelancing gig, think about all that time that is just spent planning, think about who really pays for that office at home. You're actually a lot cheaper than an agency and aren't earning as much you think.