My DS age 15 did not have any diagnosed SEN but had been through a very traumatic time with his birth mum and had some anger management issues as a result which had led to several temp exclusions from his previous school. When he was placed with me, in a different area, none of the local mainstreams would take him, we were only offered the PRU.
My experience with DS ending up in PRU was very negative. Almost all of the other pupils were there due to behavioural exclusions from mainstream school, usually for attacking other pupils or staff members. He made friends with one kid there who had been excluded from his previous school for punching a teacher. Said kid eventually got caught burgling our home about 6mths later :(
Vast majority of pupils were male. There were a handful of female pupils, most of whom had the ambition to leave school and shack up with the local top drug dealer. The male pupils either wanted to be dealers themselves or they just wanted to spend their lives smoking weed, playing Xbox and "banging hos" 
DS was there for one school year and by the end of it, three other pupils in the unit had been removed from school as they were serving custodial sentences. Bear in mind there were under 30 pupils total.
DS got into verbal fights every day and physical fights about twice a month. He was constantly trying to wag off sick because he got so stressed at dealing with the other kids. Eventually he ended up not even going into the unit every day and being set work at home when it became apparent he could not cope with attending in person.
I could not have afforded to home ed him but if I had it would have been immeasurably better for him.
I can't say a bad word about the staff, some of whom went far above and beyond the call of duty to help my DS. But they did not have the resources that he needed, and it was totally the wrong environment for him.
Sorry for that rant.
Regarding your specific question about de-registering him before he gets a perm exclusion, I would say it doesn't make any difference, because any school you try to put him in will call the other school for details and turn him down if his record is bad. My DS only had temp exclusions on his record but still none of my local schools would take him.