Asbestos is only a problem if damaged or disturbed, if you're unsure what the insulation is I'd advise to have it checked by a registered asbestos removal company or leave it alone.
There's nothing stopping you over-insulating, but you'll have problems finding the right size as even though it's 15mm pipe work the current insulation may mean you need to buy insulation for 22mm or 28mm pipes.
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