I'm almost certain you need to present yourself in person at the police station with your documents. You don't really need a lawyer or gestor to help you as it's not a complicated process.
So I'd say those ads were scams.
I had a look at some of these ads and they seem to be genuine. They are mainly law or accountacy firms and you have to give them power of attorney if you can't attend in person. Their fees are quite high (average €150 per person) whereas if you do it yourself you only have to pay the tax, which is about €12 I think.
It's not difficult to do it yourself once you are here in Spain, or you can apply in person at the Spanish consulate in London or Edinburgh. I guess it's a case of how busy you are and whether you think it's worth paying to save the effort.