Vauxhall Astra GTC 1.4 Turbo - engine overheating

My partner bought the above car about 4/5 months ago.

Everything was running fine up until about a month and a half ago, the engine temperature would sky rocket when she was driving and a message would come up on the dash saying ' Engine Overheating. Vehicle idling'. (It would also say AC off. Engine too hot, but the AC wouldn't be on).
My partner took it for an MOT and told the garage about this. They did her MOT and informed her she has a leak coming from her water pump. They didn't have any available appointments for her to book this in to fix it so we called a mobile mechanic. He came a few days later and replaced the water pump. The car was fine for about 5 days and then the warning would come up again and the temperature would, again, go through the roof.
We called a different mobile mechanic who came and ran a diagnostics test but no errors showed. He suggested that it may be the temperature sensor and so he replaced this. Again, the car was fine for about a week and then starting with the warning message and high temp again.
The same mechanic came out yesterday and took a look at the car. He said there was a leak coming from her coolant bottle and so he replaced the bottle and the cap to the bottle, and filled it up with coolant. We drove the car last night, everything fine.
My partner has driven the car to work this morning and, AGAIN, the warning has come up and the temperature gauge has also sky rocketed.
This seems to only happen when she is on a fast road (dual carriageway etc) going about 70mph and comes up when she's been driving for over 2 miles at that speed. If we are driving through a village or a 30mph zone it doesn't do this.
Sometimes when the warning comes up it will also say 'limp mode'.

We was told by the first mechanic that it could be the head gasket but he also said we would have seen white smoke coming from the AC which we haven't.

We go on holiday in about 2 weeks time and no garage can look at it before then, plus, she can't actually drive it to a garage because the warning lights and high temp keep happening so the car is virtually un-drivable at the moment.

Does anyone have any suggestions or advice please? (The mechanic who replaced the temperature sensor also doesn't do head gaskets so we are a bit stuck with that too, if it is this).

Thanks in advance.