Severely overgrown allotment, beginner needs help!

I have taken over an 139m2 plot which hasn’t been touched for over a year, it has long grass and a large area of over 8ft brambles. I was really enthusiastic but now it all feels quite daunting and I’m not sure where to start.

I’d really appreciate it if some of you experienced gardeners could give me some idea on where to start my inevitable hard work!

I was going to rotivate the whole lot after cutting back but I am worried about the perennial weeds multiplying during the process. Just feels like a huge task to fork it all up and what not! Any help and advice would be greatly appreciated as I don’t want the lack of knowledge on my part and the amount of hard work put me off from getting started!