Since the overwhelming majority of new E-Mini traders lose money it's only going to affect you if you are in the minority.
My understanding is that unless you are considered to be running a business (i.e. subject to income tax and N.I.) it's only capital gains tax - both on any "trading profits" and from any "profit" on on US$ margin funds when you convert them back to £ (if the £ were to have dropped during the period your funds were held as $) - with of course the annual CGT allowance.
So for 1 or 2 lot beginner traders it's unlikely to be an issue.