If she loved you before you got a £10K pay bump, then I think it's a little unreasonable to change her mind when the circumstances changes to £10K pay bump minus £300 per month.
If your disposable income is £600 after paying everything off and you're already over-paying your mortgage and putting into savings, then allocating £200 per month of that excess to enjoy life is not unreasonable, imo.
Of course I'm taking your word that this is what would make you happy and that this isn't just some immediate gratification that would wear off in six weeks and leave you wanting to spend the remaining £400 on fun as well. But so long as you know you're own mind and you genuinely would continue to get that pleasure from owning the car every month, I think it's your money. You seem to have met your commitments in terms of looking after your child and paying into your home so the rest is yours.
Of course a marriage is filled with "invisible" transactions. Does she feel she supports you in other ways which you're now ignoring or haven't 'paid back'? Do you have much in the way of savings? If not, I would suggest meeting half-way and putting aside a little longer so that you can demonstrate this loan commitment wouldn't cause big problems if your circumstances changed, etc. Has she or you had big problems getting out of debt before?
Life doesn't stop when you get married and a partnership shouldn't have just one person in charge of it. I would sit down and talk it through and make sure the following points are communicated:
1) The marriage and your daughter are your priority (I hope they are)
2) But that circumstances have changed and you as a couple are able to loosen up a little now and spend something on yourselves. That you're able to meet your commitments to your family AND get something for yourselves.
3) That everybody has different interests and although we may not always relate to what other people enjoy, we can still accept that it is something they really care about.
4) That you wanting to buy a car is NOT you not caring about your family's financial security, but you believing that buying the car isn't a risk to that.
#4 is probably the real point of argument. She likely believes the former and you need her to know that it's the latter (I hope this is the case). If the problem is the former, then show her it's the latter. If she can raise credible concerns to the latter however, listen to her.
5) You love them both but you still want to enjoy life a little and that's normal. There are a lot worse that husbands do. It's not a gambling problem, it's not a drinking problem, it's just a tired of driving a crappy car problem.
Finally, rent or watch The Fantastic Mr. Fox one evening with her and your daughter. You both sound like the main couple in that film:
https://www.amazon.co.uk/Fantastic-...71245594&sr=8-3&keywords=the+fantastic+mr+fox
Of course you could just be an irresponsible idiot who cares more about cars than your marriage. However, I'm hoping that's not the case.