Overheating at motorway speeds/high revs for long periods of time

The subject pretty much says it all. Basically for whatever reason when I’m at high revs usually motorway driving for more than 20-30mins at a time all of my coolant is pushed back into the reservoir which is obviously causing the engine to overheat as there is very little coolant to run through the system I have done a full coolant flush not long since my mate seems to think it could be the head gasket seal and the combustion gases are pushing the coolant out of the radiator but that doesn’t make sense why it would only happen when motorway driving for longer periods of time as it is totally fine any other time and even fine if I only get on the motorway for 15min so I’m confused. Any suggestions before I get it block tested as I’m currently on holiday?

It sounds like your head gasket has failed. I would be testing for this, pressure test or chemical test for gasses kin the coolant.
Only saying as pushing the fluid into the bottle is a symptom and head gasket failure is a Jimny thing.
 

When the coolant gets hot it expands and the excess is pushed into the reservoir. Normally you only get a bit of coolant in the reservoir, but if it is filling it up and overflowing then it’s more than just expansion, it’s getting pressurised, the common culprit is a head gasket.

If you get the engine hot, open the bonnet and rev the engine, look for bubbles in the reservoir or do a pressure test.

Another common symptom is a spark plug, sometimes one will be washed clean whilst the others have the usual deposits on them.

In really bad cases you can sometimes see the steam in the exhaust,or white smoke.

Head gaskets are usually a fairly easy ob for your local garage.

Robin

It sounds more like a clogged rad to me.  Can you control the temperature by using the heater on full blast?  Does the level in the coolant reservoir return to normal overnight when the engine is stone cold?

Head gasket failure would show up as permanent loss of coolant, steam from the exhaust even in warm weather and with a hot engine, mayonnaise under the oil filler cap, and (normally) an odd smell from the oil.

Most cars don't get serviced according to the schedule after the warranty expires, and the "mileage OR time, whichever comes first" rule is often ignored.  So many Jimnys will be on their original factory fill of coolant.  That's why things block and corrode.
 

Well I thought that might be the problem to but after a very thorough coolant flush the problem still persisted

I don't think a flush really does much as the radiator passages are so small and, if they're already blocked, they don't get flushed.

Overheating only at high loads does suggest poor cooling rather than anything else.  As well as the rad itself, is the thermostat opening properly?  How hot is the bottom of the radiator when it overheats?  Is the fan belt slipping?