No, it’s certainly not worth replacing your standard inverter with micro inverters.
(Was this a cold call? Sounds like all the other eco scams to me.)
Saying it costs only £20 a panel is a bit of a distortion. At £25 a panel (a more realistic price difference) that’s £4-500 for a typical installation. A retrofit will involve the installers getting up on the roof, removing all the panels, adding the inverter to each panel and replacing the DC wiring with AC wiring. That’s going to amount to quite a big bill for labour.
If you have a serious shading issue it could have been worth installing micro-inverters at the outset, but retrofitting a satisfactorily working system is a waste of money and resources.