I would definitely pre-book a hire car in the UK. There could be some stand-by deals if you do a tour of the rental desks at the airport, but I personally would not risk it for a medium to long car hire. Also if you are taking the car out of the mainland (across to Ireland by boat) there are all sorts of issues with insurance.
1car1 and HolidayAutos are a couple of good bucket shops that usually beat the majors - sometimes by a long margin - but I have also used Qantas Holidays and even BA and nabbed a good deal from their earlybird brochures.
Cheers,
Andrew
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