Sometimes. It can cut charges and make a plan much easier to manage, but older schemes can carry guarantees worth considerably more than the convenience of tidying up.
These are the features worth checking on every old scheme before anything is transferred. Any one of them can be worth more than the whole benefit of consolidating.
There is no number. The question is whether you know what each one is invested in, what it costs, and who gets it if you die. If the answer is no, the problem is visibility.
It can, if it gives up a guarantee, triggers an exit penalty, or moves you from a cheap old scheme to a more expensive new one. That is exactly what the checking is for.
Usually you can, but you would normally stop receiving the employer contribution, which is rarely worth it while you are still employed there. Consolidating previous employers' schemes is the more common move.
Not by itself. What it can do is let one strategy be applied to the whole amount, at a lower total cost, which is a different thing from a better return.
Every scheme gets checked for those features before anything is recommended, and where the answer is to leave one alone you will be told so plainly.
Tell us what is on your mind: a pension you have lost track of, a fixed rate ending, a will you keep meaning to write. We will tell you honestly whether we can help.