score:4
If your phone is Android, you can download MapFactor or one of many other apps for free which allows offline navigation based on the maps from the OpenStreetMap project. Be sure to download the maps (also for free) via Wlan before you need them.
Upvote:2
I would highly recommend getting a GPS over using mobile with data. From my experience, mobile signal is sketchy outside of the city. Also you don't have to worry about your phone battery etc.
Try some companies that just rent GPS devices (on the cheap):
Regarding GPS from the car rental company:
The cheapest car options from most car rental companies in the UK, are still new models and often come with GPS albeit disabled. Call them up to see if they incl GPS for free or if its an extra. I've experienced both cases where we skipped on the GPS only to find the car came with the GPS for free. The other time it was disabled.
Upvote:4
If you currently have a GPS device, you may be able to load OpenStreetMap based maps on it for free. eg this is possible for most models of Garmins.
For getting OSM maps on a Garmin, easiest to use this website: Garmin.OpenStreetMap.nl. You can pick which country you want, and it will give you a file to download. Depends on your device and how much memory it has, you may be able to load maps for a number of countries at once. Or you could put maps on separate SD cards, and swap them around as required.
OpenStreetMap generally has good coverage for roads in most of the UK. Plus it has a lot of off-road details, eg footpaths, and lots of points of interest, eg shops, pubs, hotels. OSM can be regularly updated, so often has newly built roads not yet shown on any other maps.
Upvote:7
Along with this question, you can get a Data Sim from O2 for GBP10 that offers 1GB of data. That should be enough for quite some time of navigation I would think, and it's cheaper than any of your listed alternatives.
The additional advantage is that you will not have to pay for internet in the hotel or elsewhere to do your email or send some photos home. If you put those costs into your budget, this method might pay for itself, depending on how much the hotel charges you for internet or if you are willing to always scout for free access internet.
On top of that, you can use the cellphone to navigate you while you are walking through the city more comfortably than carrying a GPS around with you.
Along with EdmondYeung99's suggestion, it's of course highly recommended t equip yourself with a car usb charger!