Under CASS regulatory rule book which adjudicates how client funds in financial institutions should be treated, your funds are segregated from platform assets and therefore, those client funds cannot be used to reimburse creditors in the unlikely event of insolvency.
In the event of insolvency, and FXCM (for example) cannot satisfy repayment claims, you are eligible for compensation as per FCA regulations up to and including £50'000.
I am, however, not sure whether this applies to a "spread betting" Forex account (such am account exempts you from income and capital gains tax obligations). I would think such regulations pertain, but this is something you will need to ascertain for certainty from the FCA, whos number you can glean from Google.