There are numerous UK-based charging networks, each with their own foibles and peculiarities. Each has different pricing structures; some are subscription-based and some are PAYG, and many aren't available where you want them! There are various apps out there like ZapMap which consolidates the various networks onto a single map, and shows you which ones are suitable for your car, which ones are reported as working and which are available.
But before you get too involved, you need to work out if it's actually worth your while to charge. Assuming you're mid-journey, you'll probably want to rapid-charge rather than wait 4-5 hours to recharge the battery via the Type 1 socket. This means using the Chademo socket, which only charges to 80% in 20 minutes or so to preserve battery life. So you'll get another 20 miles (or less) of range. That would cost around £3 in petrol. How much would it cost to recharge the battery? Well, it depends (of course!), but you might be able to get it for less than £3 if there's an available (and working) charge point where you need it. On motorways in England, it's often Electric Highway, which sometimes vends free, but otherwise costs 30p per kWh, which would be cheaper by around 60p
So would it be worth the hassle of finding a working, vacant charge point to save 60p? Unless you can find one on free vend, many people would say it's not worth the hassle. YMMV