IB -
The minimum balance of £1450 is required to open an account. It is also required to open new positions, but it can fall below that as a result of an already opened position, as long as the position does not fall short of the specific margin requirements applicable to that instrument.
Per contract for YM you need $1250 initial margin and then $1000 maintenance margin
These margin minimums double after 8.45pm GMT.
Open positions will be closed if you go below margin requirements.
You need the initial margin to open a position. But once in a position is open you then only need the maintenance margin. So though you need $1250 per contract to open a position, you only need $1000 per contract to keep it open.
So you could trade 2 contracts with $2500 and then keep them open with $2000. This is good news as it means you can trade 2 contracts, nearly 3 in fact, with £2000! I am not suggesting you do this, mind.
But if you lose enough to bring your account balance below $2000 they will be closed for your protection, then you'll be below the minimum balance and won't be able to open any new positions.
I have not heard of the $500 offer and don't believe they'd allow it, but I may be wrong. GBP is the way to go then you can choose whether and by how much to hedge your account against movement in the dollar (by buying and selling dollars).