Ok so after 4 weeks of being at the garage the car is back now and working again! Just going to post what happened so it helps other people in the future.
So after I sent the car to the garage, they were looking at changing the start button on the dashboard. Turns out this didn't fix it as the 12v battery was dead the next day. Then they started pulling fuses to see if it helped. Pulling the ECU 2 (the body control ECU) fuse turned seemed to do the trick and the radio/ac/lights went off. Plugging it back in brought the issue back.
So they phoned me and told me that and quoted me £1050 for that ECU alone :(. They then left the car overnight with the fuse off but this still killed that 12v battery again, and they were sure that it was something else(even after replacing the ecu with a new part so they put the old one back)
After a few weeks of head-scratching and basically getting nowhere, the car decided to drop a hint of where the issue was by activating the headlight washer and emptying about 3 litres of washer fluid all over the shop floor. This was a bit suspicious so they looked closely at the headlight washer and pump assembly and PRESTO!!! The Ecu controlling the headlight washers(different to the windscreen washer ECU) had basically had a short circuit, and made the washer pump run 24/7, which also burnt out the motor in the pump when it ran dry. Was quoted £700 just for those two parts to fix it(and it was very fishy that they had the parts in shop already) so I just said **** it and they unplugged that whole assembly and the car has been fine since I got it a few days ago. I think I will wipe the headlights clean myself rather than pay £700 for the stupid headlights to get a stupid spray on them.
All in all it was about £500 plus vat for the diagnosis including a new battery as the 'old' battery had died being discharged about 10 times during the last 4 weeks. Basically the issue was with the headlight washer ecu and assembly, and im grateful to them for finding it cuz I would not have in a million years thought it would be something so simple yet so frustrating to find.
Anyways, I hope it helps someone out there who comes across this issue in the future, start pulling out fuses and if it is the body control ecu 2 start looking at what is connected to that ecu, especially the headlight washers as Lexus having the parts on their shelf already means its probably a more common issue with these basically bullet proof cars.
Going forward, I think its time for me to sell this car as much as I've loved it. Its appreciated in value quite substantially since I got it and with a baby on the way, I cannot justify spending at least £500 every time ive had an issue with it(like 5 times already). will settle for a cheap Honda until I can come back to Lexus and own my dream car, the LC 500.
Thank you all for your help in trying to find this issue for me and I hope it proves useful to someone in the future 🙂
All the Best,
Josh