I’m not up to speed on Enyaq battery sizes, is it the 77kWh one you’re looking at?
The most you’ll get over that 4 hour Go period is 28kWh. I’m ignoring charging losses here, no charge is 100% efficient.
So from zero charge, 28kWh at 5p per kWh is £1.40. That leaves 49kWh to go, at 15.91p per kWH which is £7.80.
Add that together and you’re at £9.20, you’d probably need to add 10% on top of that to cover charging losses, not everything drawn from the wall goes into the battery.
Compare that to 24/7 tariff like the one I’m on at circa 12p per kWh, and the same charge would cost £9.24, plus charging losses.
I’d you’re not using the entire battery every day, then Go is a good tariff, but if you need the flexibility of plugging in whenever you want/need to (for Taxi work?), then a cheap standard tariff might work better.