You are attempting to engage in currency speculation and you will lose. The deck is stacked against you. First, if tie up a 1000 US you lose the interest on that money -- maybe 30 or 40 dollars. Second, if you buy pounds on this side you buy at retail and will pay a premium ranging from 5 to 10 %. So your initial 1000 may have shrunk by a 100 plus dollars. So just to break even in one year you would have to have dollar decline of at least 10 to 15%. That would be a significant swing. Lot of risk
You mentioned using a cash card. (I am assuming you do not mean a debit/ATM card) Be very, very, careful with cash cards. Historically cash cards have had a lot of undisclosed fees associated with the card. Be sure you understand the fee structure AND the exchange rate used. Cash cards bury their fees in the exchange rate.
It has been repeated many, many times on this site and others, that a debit card tied to a checking account is the most economical AND CONVENIENT way to obtain foreign currency when traveling.