Sorry to be pedantic, but I'm going to be anyway ..
LASER cards don't work in NI/GB. If your card has a Maestro or a Cirrus logo, they may be accepted as such, not as LASER. If your LASER card does not have these logos, it won't be accepted so, for that reason, it is not accurate to say that LASER cards work in NI/GB.
Although LASER is a generic term for your bank debit card, it's not always only a LASER card, in the same way that that not all vacuum cleaners are Hoovers.
Seperate to the rate, you'll also get hit with a seperate cross-currency transaction charge which, in effect, increases the price of your GBP. Similar charge would apply for credit cards if you opt to pay in GBP.