Most roofers will look at your roof and give you a price without charging for it. Some charge for a written report. Both are normal, and what separates them is what you have asked for.
Nobody publishes reliable data on this
There is no authoritative figure for how many UK roofers charge for a quote. Checkatrade, Which? and the trade bodies do not publish one, and the percentages that turn up on roofing blogs are sourced to nothing. A page telling you that "most roofers charge" or that "only 15% do" has made the number up, and saying so is more use than joining in.
What can be said is that practice is mixed, that free estimates are the norm for straightforward work, and that a fee usually signals a different product.
A quote and a survey are not the same thing
A quote is a sales document. The roofer looks at the job, works out what it will take and gives you a price, in the hope of winning the work. Producing it is a cost of selling, which is why it is normally free.
A survey is a professional opinion. Someone gets onto or over the roof, inspects the covering, the flashings, the visible battens and the state of the underlay, and writes down what they find whether or not any work follows. That is a deliverable in its own right and charging for it is reasonable, because you are buying the diagnosis rather than the repair.
The honest line is that a quote should be free and a written condition report reasonably is not.
What a paid inspection costs
Written surveys and condition reports run roughly £50 to £300, and Checkatrade prices a formal roof inspection at £250. Those figures exclude VAT and exclude access equipment, in line with published cost guides generally, so a roof that needs a scaffold or a powered platform to inspect properly will land above the headline.
Many roofers credit the fee against the job if you go ahead with them. Ask whether yours does, and get the answer before the inspection rather than after it.
When paying for one is the better decision
A survey earns its fee when the question is diagnostic rather than commercial.
If you are buying a Victorian house and the homebuyer report says only that the roof appears serviceable, a roofing-specific inspection tells you whether the underlay and the nails are at the end of their life. Those two components, rather than the slate, are what force a re-roof on a South London terrace, and a general surveyor will not have looked at either.
If you have a leak nobody can locate, a report from someone with no work to win is worth more than three free visits from people who each hope to sell you a roof.
And if your quotes disagree wildly about scope, an independent opinion settles what needs doing before you try to compare prices. Two quotes for different jobs are not comparable however carefully you read them.
A free quote is still costing someone
A roofer visiting your house spends an hour or two travelling, inspecting and writing it up, and wins only a fraction of the jobs quoted for. That cost sits inside the price of the work they do win. It has not disappeared; it has been spread.
That is worth remembering when you line up a sixth opinion out of curiosity. Three written quotes on identical scope is the standard advice because it is roughly where the information stops improving, and a roofer who suspects a job will never proceed is entitled to ask for a fee to look at it.
Settle it before anyone climbs a ladder
Ask whether there is a charge and what it covers. Ask whether you receive a written report or only a price. Ask whether the fee comes off the job if you proceed, and get that in writing rather than as a friendly assurance on the doorstep.
Then ask the questions that make the resulting quotes comparable: whether the figure includes VAT, and whether scaffolding, waste removal and any pavement licence sit inside it or arrive later.
Paying for a diagnosis is a fair trade; paying for a sales visit is not.