To re-wire or not to re-wire in a rented property

Other

What is the general rule for re-wiring a property when it comes to being rented out?
How do I know when it should be done versus it is still acceptable for a few more years? How do I know when an electrician is just trying to scare us with regulations so we pay for something uneccessarily?
Our sockets are in the skirthing boards from the 1930s. I believe the re-wiring might have been done 20 years ago but inherited from parents so not sure. Is there a way of knowing when it was done by looking at the wiring?
There was a test done and a heat sensor installed in 2007 so the wiring was checked then and I believe the fuse box was changed then.
What do I need to have in mind when deciding if it should be done and what do I need to be aware of when I have electricians around? Can you use someone who is not certified if their work gets checked by someone who is certified?
If I was to consider doing solar panels and or installing heat pumps should I do this before a certification or will it be okay to do it after?