You can either get one with UK (or full European) maps built-in, or you can get any model that has an SD card slot and buy the UK/Ireland map add-on for about $40. I think the current models that are preloaded with full European maps are the 2559LMT, 2577LT and 24475LT.

Although if you'll have a smartphone with data access in Europe, Google Maps/Waze can be better than a Garmin these days because they integrate real-time traffic information that's much more sophisticated than the traffic data available to Garmin devices (and not all Garmin devices even do traffic).