H6s are not unduly prone to HG failure but if it's overheated, all bets are off. Firstly, when you first got the coolant light did you check the levels in the radiator and the expansion tank?
Has the temperature gauge risen at all indicating overheating? These engines normally run with the temp indicator steady at 9:00 on the gauge. Does it push coolant into the expansion when warm/hot and if so does it draw back when the engine cools? Pressure tests can be ambiguous as are combustion gas/coolant tests. There is usually no mixing of oil/coolant. History of the car? It could be as simple as not properly burped or the wrong thermostat, Subarus are fussy about using genuine Subaru stats.
If it ends up being the HG, very complicated to do so £4000 is possible.
I wouldn't use HG sealant (Holts Radweld or similar) unless you are really desperate and don't mind small coolant passages getting gummed up with crud.