Please don't tell me it's the head gasket

[Bowline5]

My 07 Forester has the EJ255 engine, 2.5l 16 valve turbo, on the way home a few days ago the temperature gauge shot up to the maximum. I limped the last few miles home, next morning the only obvious issue was a very low coolant level in the expansion chamber. I topped it up, everything is fine; no leaks, no clouds of steam or white smoke, engine smooth and pulling well BUT in a little less that 100 miles the expansion chamber level had gone from full to nearly empty, so where is the fluid going? The car had no service history when I bought it 3 years ago, it had barely moved in 3 years, it's done 173000 miles. If the gasket is failing it would be daft not to do timing belt, plugs, pulleys etc etc at the same time, US sites suggest parts and labour at $3000 to $4000 dollars. Does anyone have any idea what the UK cost might be?, or where there is a good aftermarket specialist who might look at it?

[Mr B]

It highly likely head gasket the issue, check some basics such as water pump and thermostat housing/hoses for leaks and also radiator and turbo cooling lines (visible coloured staining marks left from antifreeze)
You can use chemical test kits detect combustion gases in cooling system (we use gas analyser over rad filler neck)
It done some miles and to be honest I wouldn't want do just gaskets at that mileage, bearing shells, all new seals, new oil pump , new rings/pistons and valve overhual tends be best way forward but you could indeed do just head gasket and basic head overhaul if the short block seems decent enough and assemble with new water pump and quality japan timing kit (ICP do these at sensible money)
Good parts and good effort won't be that cheap & at this mileage further issues could crop up from fatigued engine stressed from the poor cooling/overheating .