Even though the UK is part of the European Union the British pound sterling is still the official currency. One can use British currency in Northern Ireland but do not expect the Irish pound to be readily accepted in Britain. You may find some stores willing to accept euros but these are primarily in the south of England.
Best way to exchange money:
In my experience, the best place to exchange money is to use a bank ATM machine. Make certain your bank card is of the four PIN number type. Most ATM's in Ireland are compatible with the Cirrus or Plus system; if you have any doubts about the usability of your ATM card, contact your bank prior to leaving. You might also want to check to see if your bank applies any additional fees for international exchanges.
When arriving at the airport, Combo/Exchange windows are very easy to find. You will need to look a little for the ATM machines. Also, make certain you have a backup method to obtain cash (credit card) should your ATM card fail to work.
Banks are generally the best place to go for exchanging cash into euros.
Credit Cards are another way to exchange money in Spain, either by withdrawing cash or by making a purchase. It is a good idea to check your credit card's bank policy for international exchanges fees and cash money withdrawals fees. Even with fees, Credit Card purchases usually (but not always) provide a better rate of exchange than a street cambio (exchange) vendor. Note - there are two levels of fees, VISA and MASTER CARD charge a 1% (I believe it is up to 1.5% now) fee on all transactions but some banks then add their own 1-3% fee. There are banks that charge no additional fees for credit cards use over seas, it is these, hard to find, no additional charge banks, you want to get a credit card from.
In Ireland I encountered, for the first time, restaurants and a car rental company providing (but not asking first) credit card bills in US dollars. Usually, it was presented as "saving me the credit card fees associated with a euro credit card transaction". In fact the restaurants and rental company made the conversion to US dollars at such a high exchange, it cost me money. In the case of the car rental, it cost me $30 more for the rental than if I had paid in euros. I did not encounter this practice in the UK but I would not be surprise it occurs.
Travelers Checks - In these days of ATM's and credit cards, Traveler Checks are not very usefully. If the Traveler Checks are in the currency of the country you are visiting, you might find they are accepted by individual stores, but even this is become rare. As back up money for emergencies, Traveler Checks may still have a roll. The best place to cash or exchange Travelers checks are in the offices of the company that issued them, American Express, Thomas Cook, etc. Finding Branch offices of the issuing Travelers Check is not always convenient if not at times impossible. Banks will cash Traveler Checks for a fee. The newer Traveler Check card may be a better option, I have not really looked in to them, but I believe they work like a prepaid credit card.