OK: thought this through and it appears the discrepancy comes from calculating the floating balance at each bar and converting it into pounds using different exchange rates for every bar. The final profit and loss is just calculated using the difference between entry and exit price and the exchange rate at the point of exit - so there’s the discrepancy.

Any ideas on how to correctly calculate the floating balance in pounds or to make the account value and PnL match up after a trade would be much appreciated.