This means that you need to convert the currency from Pound Sterling to US Dollars. In todays case, 1 Pound = 2.03 US Dollars ..so 250 Pounds x 2.03 (pound - dollar exchange rate) = 507.5 US Dollars
pound for pound is a way of saying without the obvious weight differance, is this guy who's 140lbs a better fighter than this guy who's 240lbs, it's who would win hypothetically if weight didn't...
Absolutely not ! The pound sterling is the currency of the United Kingdom, whereas sterling silver is the percentage of silver put into an item when it is made, like a coin or jewellery for example....