This adjudication from the Advertising Standards Authority (ASA) is interesting.
All of these test booking services (all the ones I have seen, anyway) are absolute scum. If they were 100% honest, even the good ones would say:
We are not the official booking site, and we have nothing at all to do with the DSA. We charge you extra for the test we book for you, and for that you get access to online training materials.
The problem is they don’t. They deliberately make themselves look like the DSA’s site, and any disclaimer is carefully hidden as deeply as possible so that those using the 
service have almost zero chance of spotting it before they’ve entered their credit card details. No one reads the fine print in detail, and even if they did they probably wouldn’t understand it. You’re talking about 17-year old kids, and the pond life operating such sites knows that full well.
In this particular case, Book Your Practical Test Online Ltd. (BYPT) tried to squirm out of the complaint by arguing that their site didn’t look like the DSA one, because the DSA “did not use orange and white on their home page”. Fortunately, ASA were fully aware that until GOV.UK came along, that’s EXACTLY what the DSA site looked like. Orange and white. BYPT even tried to argue that their logo had an arrow that was different to the one the DSA used – trying to avoid the issue of why they’d even got a logo that was so similar to the DSA/Directgov one. Oh yes, they knew exactly what they were doing.
Fortunately, so did ASA, and it upheld the complaint in full.