.....and don't forget the charges:
Amounts up to £500 = £10 one off charge
Amounts £501 - £750 = £12.50 one off charge
Amounts £751 - £1000 = £15 one off charge
Amounts £1001 - £1500 = £20 one off charge
Amounts £1501 - £2000 = £25 one off charge
Amounts £2001 - £3000 = £30 one off charge
Amounts £3001 - £10,000 = 1% charge
....they win both ways: they keep your cash (and all the interest) until March, and they can bag the currency as and when the rates are at their best.
Personally, I keep my money in a high-interest savings account and use the Post Office CC whilst in the US. This gave me rates of over $2 to the £1 in August and I didn't need to pay a penny until I got back from my holiday, meaning my cash was earning interest for as long as possible.