It's very hard if not impossible to change to a US broker based in the US if you are not living in the US. I tried with multiple brokers. Ever since Sep 11 2001 they clamped down.
I feel lucky I can trade with IB (as Ameritrade, Schwab etc said no way if not American or living there). I just have to change from IB UK to IB US. Pain in the a** though and the European regulators are as dumb as.... Good on the UK for leaving.
Try Tastyworks and TradeStation. They offer international accounts for non-US traders.