You've highlighted a few things that may be causing him difficulties at school then.
"what we do at home is relax and take it easy - there no rigid routine that he must follow as he does at school"
"We do have to give him verbal warnings for coming off the computer, eg, ten more minutes, but sometmes he'll still have a strop about this."
"We often loose him in supermarkets because he wanders off"
If there is no routine at home, he may be finding it hard to conform to the structure and expectations at school. I'm not suggesting that you have a rigid routine at home, but your ds does need to understand that what happens at home, is not the same as at school.
It's not always possible for a teacher to give individual or even group pre-warnings of a change in activity. However, this may be what needs to happen at school to help your ds transition between activities.
He wants to do his own thing in supermarkets and is unaware of what others are doing, or are expected to do.
I can see why these things could cause problems at school. I think you really need to take the school's concerns seriously. Saying that you hope secondary school will suit him better is not good enough at this stage in my opinion. He's 10 and needs support to access school appropriately. If he gets lots in supermarkets - imagine the potential when he's moving between classrooms at secondary school.
Behaviour such as turning over tables at 10 and hiding underneath them is pretty extreme in my opinion (and I work in an incredibly challenging school).
If a child turns a table over in my classroom, I may need to remove the rest of the class to ensure their safety. This would obviously disrupts their education.
Continued disruption of the class, and putting others at risk would eventually necessitate putting a child on a fixed term exclusion in my school.
What agencies (other than CAMHS) have the school suggested involving?