Cheaper to convert cash or use card in NI?

I have an Xmas present to buy which I will buy in NI. It costs around £450 STG. Which would be the cheaper way to buy it?

1. Convert Euro to pound at exchange bureau and pay for it in Cash?

2 Pay by Mastercard and then pay it off next day online banking?

I understand the interest etc, but does it outweigh the lower exchange rate at the bureau?

before you head north, you can check the rate online for the bureau de change in newry buttercrane (no connection, but i find them handy) http://www.bureaudechangebuttercrane.com/ you can also ring them and book money if you are changing a large amount.
you could also check where you are buying the item and see if they accept euro and what their rate of exchange is.