Have you replaced any British words with their American versions?
For example, switching out the word 'boot' for 'trunk', or ditching the word 'rubbish' for 'garbage'.
This is something I've noticed my 6 year old does pretty regularly. We went through a stage where 'sweets' became 'candy', 'holiday' became 'vacation' and 'courgette' became 'zucchini'.
That last one didn't happen but if you're still reading you've got my respect, or as the Americans might say '...mad props'.
I don't think I've ever heard anyone say it IRL. It's a pretty minor thing really, use whatever insults you fancy, but if you want something very British you've got classics like muppet, nonce, pillock, bawbag.
I'm partial to 'wankspanner'. Which is pretty un-American since even if they knew what to wank meant, they'd probably to with 'wankwrench' which just isn't the same. Sorry, I'm moving into rambling territory now.
Honestly, bellend is a wonderful insult in my opinion. I used to watch a lot of sovietwomble and similar youtubers in the past so that and using cunt like a more colorful version of calling someone a dick were something i heard a lot and have a lot more behind them when said. That being said, I live in the US and would have to explain Bellend which would cause it to lose the impact. And cunt is far more negatively received here so you will likely be regarded as a mysoginist and/or get clocked here.