Depends on availability of time skills and money . From what I gather its a rarely used vehicle and while you would stretch to replacing the head gasket if that was sure to cure the problem , you have no intention of replacing the head , and so it seems the end result is either to pull the engine apart on a 50/50 a new gasket will do it or the truck goes to scrap . Or you keep driving it until it stops then scrap it .
If I'm correct on your current thinking then what harm can a £20 experiment do ?
Flush the coolant , disconnect and block heater pipes , remove thermostat , fill it up with water and run the engine up to temperature .
Bar's head gasket stop leak interests me because unlike most of the others which use copper particles to block , the Bar's is some sort of resin , so while having no idea how they actually work (or don't) I can imagine a resin sort of coating everything .
When engine is hot pour it in your radiator and fast idle for 20 minutes then shut it off and replace your thermostat the next day .
Might fail or it might buy you some time . I'd never consider it a fix but I don't see what you have to lose .