I have a 2004 MK6 Transit which runs at half temperature on the dial. The needle went to 3/4 so I pulled over and checked the expansion tank to find no water but full of oil and the engine had no oil on the dip stick.
I replaced the Oil Cooler, replaced oil filter, filled with 8 litres of oil and started flushing out the water pipes. The oil was only on minimum on the dip stick so I went to get more. The system still had a little oil in the water but was nearly all drained. I have drove it for just over 25 miles with heaters on hot to circulate and try clean all the pipes. I got back from a 25 mile drive and drained and refilled the water pipes again then took it a mile up the road and drained it again but this time instead of the water coming out the radiator being nearly clear it was the colour of a milky drinking chocolate.

I have added an extra 5 litres of oil and it was showing full on the dip stick. The oil has clearly gone straight in to the water again. Took it for a drive and after 13 miles the pipe across the top of the radiator blew off and covered my engine in oil and water.

Is this is a head gasket problem or something less severe?

I suspect that  because of the oil in water its more than likely as you suspect the head gasket or more unlikely( probably not) a warped head if it has overheated.In either case its fixable .I would get it checked over at the local garage to be sure before its to late. The blowing off pipe and oil in the water  sort of says it all ( the extra pressure in the system).This is only my opinion but do get it checked in case.