I don't know if you leave far from "biggest town" because near Biggest town there are fuel stations with ADBLUE pump.
At the beginning, those stations were only used by trucks engines (first Euro 6 engines).
The station manager told that small engines can't use the station because the Adblue flow is very quick (ad blue for trucks has a capacity up to 20 gal) but some people fill their plastic bottle because it's really cheaper (about 0.50 € per litre --> 2.27€ per gallon). Adblue is the same for all engines (Trucks or small engines)
Nowadays, because Euro 6 becomes more and more frequent there are now adblue pump with less flow for small vehicles at the same price.
I live in a very small town and the Adblue station is 30 miles far from my house
I don't know if in GB you have the same adblue pump in fuel stations