I'd recommend having Electroverse, Shell and Possibly Elli RFID cards.
Shell charge a roaming fee for using a non shell charger, but there's no monthly cost so it's still worth having as a fallback.
Waving an RFID infront of the charging post is usually less of a faff than messing with apps and the ones mentioned above are the ones I know of which support multiple charging networks.
Of course Murphys Law will probably dictate you end up with a charger that needs it's own app anyway.
Edit: Also if spending Time in Scotland a chargeplace scotland RFID is worthwhile.