A lot of people looking to change a worktop without changing the kitchen find overlays first. It is worth knowing what the difference actually is before you price either one.
An overlay bonds a thin layer of stone or composite over the worktop you already have. It is quicker and it is cheaper. What you keep is everything underneath: the old surface, the old substrate, and any water damage that has been quietly swelling the carcass under the sink. The finished top also sits proud of where your worktop used to be, which shows at the ends of a run and against an appliance.
We do not fit them. That is a choice about what we are willing to guarantee, not a claim that nobody should ever buy one. If an overlay is right for your budget, take one, and take it from a company that fits them properly.