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

Speaking from experience you will find that when you have replaced every other part of the cooling system because you don't want to face the prospect of a failed head gasket that is what it will be. The m13 is not prone to overheating but when it does it will take the head gasket with it. It will also probably need a skim too. Been there, got the T-shirt.

They don't have a massive reputation for blown head gaskets - it's not a Rover K-series* - so it needs some careful diagnosis first.  There's another thread on here where it turned out to be a clogged rad (that one even went back to the main dealer for diagnosis, and thankfully they didn't leap at the chance to do a £2000 job!).  A few other people had had the same experience. 

*usually in the MGF, because the location of the engine hides a small coolant leak that they are prone to

 

Will any garage be able to do a proper diagnosis and how much would that cost roughly. As at this point I’m questioning whether it’s even worth the hassle of fixing.

It's always worth fixing.  Have you seen the price of Jimnys now?  Only severe rust can write them off.

A blocked /  faulty radiator is easy enough to eliminate. Go for a run and get up to temperature, stop and feel the radiator top to bottom (engine off). It should be a uniform temperature to the feel, any cold spots are a blockage or delamination of the cooling fins.
Unfortunately blown head gaskets on the Jimny is a reoccurring theme in temperature regulation problems, much like swivel oil leaks are the Kingpins. It's hard to say if the head gasket is the cause or the victim of overheating (chicken or egg), but as Lambert said most end up fixing the head gasket. So eliminate this first as it's high in the probability charts.
Any garage should be able to test, you can even get DIY chemical test kits online to check for exhaust gasses in the coolant.
Is it worth fixing ?
Depends how good the rest of the Jimny is and how much you want to keep it. It's not a complex job, again most garages should be able to provide this service and it's easy enough to do yourself if you enjoy working on vehicles. If you don't pay someone.
Have a feel of the radiator and good luck.

Here is a DIY test kit. Shop around, cheaper exist.
Test kit

Oh and it's not a 2000 pound job. I had the head gasket and clutch done at the same time with the engine removed and that was only about 850 pounds. But in the preceding 3 years that I spent nursing the car along thinking that the next step would fix it I probably spent most of the 850 on parts and labour trying to replace radiators,  water pumps, viscous fan units, thermostats and hoses and numerous flushes and bleeding attempts. And then I still had to do the gasket.  Now I had had that one from new and at the time had no intention of shifting it so it made sense to preserve with it so I saw the whole thing through,  most people would not be keeping it that long so would put a new radiator in deciding that should fix it and then sell it on without ever finding the full extent of the problem.  But believe me once they start having temperature control issues they will need a head gasket.