It all depends on the exchange rates, which are always fluctuating. Banks also charge commission for making an exchange, so you need to take that into consideration, too. At exchange bureaux, who may also charge for exchanges, they always display the rates of exchange so that you can shop around for the best deal and newspapers publish the daily rates. The internet is also a good source for this info. Today, 21st April 2015, the rates are £1=$1.49 (USD) and €1.39. This means if you have €100 you will be able to buy 100/1.39 GBP=£71.94; if you have $100 you will be able to buy 100/1.49 GBP=£67.11. So if you have €100 and $100 you will be able to buy more GBP with the euros, because the euro is more valuable. You would need 1.49/1.39=1.072 USD, about 7.2% more dollars than euros to buy the same amount of GBP, so you would need about $107 to buy the same amount as €100. To avoid exchange charges your are best to use your own country's currency to buy GBP rather than convert it to dollars or euros and then exchange it again for pounds.