As mentioned, there are no pronunciation police to haul you off if you mix your styles - but as a non-native speaker you will already have a 'trade-mark' accent that people will judge your ability on [whether they mean to or not].
Because of that, I would pick a style & stick to it.
It's quite likely, depending on the strength of your trade-mark, that native speakers wouldn't notice the differences if you switched from one to the other anyway.
I think taking that trade-mark & adding further idiosyncrasy would be counter-productive, so...
US slang "I wahn-ned a bedder solution" - avoid.
British slang "I wan`ed a be`er solution" - also avoid, unless you feel truly confident in your glottal stops.
US or British articulate pronunciation "I wanted a better solution" - stick to that. Put the t's where they belong.
Having said that, we don't know what your native accent/language is.
In hugely broad terms - if you're Northern European people will probably think you speak with an Americanised accent anyway. Southern European or really anywhere further afield, people will have far less certainty. If you're Asian or South East Asian, most un-travelled people won't be able to tell which you are aiming for most of the time.
People who have themselves travelled a lot will always have an easier time of it when communicating with another traveller, wherever they're from. It's just the way of the world.