It can be a bit confusing selecting a number plate website. There are a lot of websites which sell number plates and so it can be hard knowing which one to choose. A good way to tell between them is by looking at the website and how professional it looks. Just by having a scan of a website you get an idea as to whether you think it is a good place to buy from or not.
Then you will probably want to compare the prices between the different websites. Lastly, you may want to look at some reviews of the websites to see what other customers say about it. However, it is important to bear in mind that anyone can write a review and so it may be written by someone who works for the company or someone they have paid to write one and then be very good. On the other hand, it may have been written by someone who works for a competitor and so they may make it sound extra bad.
It could be a good idea just to trust your own instincts on a website and company and hope that they are right. If you shop a lot online then you will know what sorts of things to look out for, what things make a good company and what things do not.
If you do know someone who has bought private number plates on lone then they will be able to tell you about the company they used and they will have no reason not to be honest and so will probably be the best way of telling which will be the best.