I'm doing a road trip from UK to Romania this summer but I don't have a GPS. I'm looking to use Google Maps on my iPhone. How could I get mobile data (I have a UK mobile)? I of course would like to avoid roaming charges but would it really mean I'd have to get a SIM card for every country I'm going through? Or is there perhaps another way of doing it?
There are various offline map solutions for iOS. I use ones based on openstreetmaps such as osmAnd. With these types of apps, you can download maps for the areas you intend to visit either before hand or at places where wifi access is available. Search the app store for "offline maps" to see other possibilities.
For mobile data - as of the 30th April 2016, by EU law, you can only be charged a small excess (+ €0.05 /mb) on what you would normally pay. From 15th June 2017, this will advance further to not additional charge from what you would normally pay. It isn't clear to me if this includes included minutes, texts and data allowances as part of a monthly contract or top-up incentives however. So if you're happy with those prices, you could use your existing contract.
Alternativley, you could seek out a mobile provider that already treats all of the EU as the same as the UK with no raming charges. Three is the best known of these in the UK (and they cover more than just the EU!), however a quick google search (and your comment) indicates that Tesco Mobile also offer this service.
Of course, @Berwyn's suggestion of just downloading the maps before you go is also useful.