If you want to argue it in court, by all means, that is your right.
That’s true, but comes at a great risk. If you wish to plead not guilty to an offence you will go to trial in a Magistrates court. To stand any chance of winning you will need to instruct a good lawyer – and they aren’t cheap (those who self represent will find it much harder to get a not guilty verdict). If found guilty, in addition to getting a bigger fine than you might have done by not contesting it, you will incur much higher court costs. To give an example, guilty pleas usually have £85 court costs. Not guilty pleas, if found guilty, come with court costs of at least £775 and possibly more. To contest these things you have to be 100% confident that you were not guilty and have evidence to prove it.