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it's possible the uk tax man could ask them for your records but you'd have to be doing some pretty big numbers. how verified are you on both of them? both allow little or no verification if you're trading small amounts.
Both require name, address + email verification for basic level. That's enough to trace you if they wanted to. You could always provide fake details, but then if something goes wrong with your account, you could be asked to provide proof of identity or lose access to your funds.
Polo has $2k daily limit, that gives $730k/year, so rather significant.