We brought our own Garmin with us last year for our month in the UK. I bought a UK SD card on Amazon (I think it was around $30). I added addresses of things we wanted to see (and, our hotels!), beforehand, and it worked just fine.
We also had a terrific UK road atlas (also on Amazon) called "Easy Read Britain 2014" that I HIGHLY recommend!: http://www.amazon.com/Easy-Read-Britain-2014-Publishing/dp/0749574526/ref=dp_ob_title_bk
It might seem a bit pricey at $22.45, but it fit easily into our carry-on bag. Before we left for our trip, we noted, highlighted and put Post-It notes about our planned route and everything we wanted to see in it. We used it a LOT. The map pages are a good size (our eyes aren't what they used to be...) and a legal clip held the pages open easily. No mad scramble trying to remember what was what or where, while we drove! I was the navigator (hubby did the driving) and the map had more things than the GPS. Like I said, I really recommend this atlas!
And, as a side note, when we picked up our car in Cardiff, the rental guy asked us if we wanted the GPS. We told him we'd brought our own. When he showed us the car and was explaining things, he said, "Oh, it has the GPS anyway, so you're fine!" We double-checked that we weren't being charged for it and, thus, had a back-up GPS if we wanted it, for free.
So, double check with the car rental agency that you aren't charged for a GPS you don't want!