IMO, if someone is charging £50 to apply a ceramic coating to a Golf-sized car, I’d say avoid it. Sorry, I don’t know of anyone in your area, but for a good quality job done well, expect to pay much more than £50.
To do the job properly, a professional detailer would probably need your car for a couple of days. The car would need to be fully decontaminated and the paintwork corrected by machine polishing to ensure the paint surface is fully clean and free of contaminants and any imperfections such as swirl marks before it’s ready for a ceramic coating to be applied. Most ceramic coatings also need to be left (overnight) to cure in a dry, dust free environment. For £50 you won’t be getting a couple of days effort from a professional detailer; you’re probably just getting someone applying a product to the paintwork with no preparation at all (outside exposed to the elements, rather than working in a proper detailing workshop), and IMO the preparation stages are absolutely key to the longevity and durability of the product.
Have a look on some of the professional detailers websites to see what they would charge to decontaminate and correct the paintwork and apply a ceramic coating - that’ll give you a good idea of what you can expect to pay, and it’ll be quite a bit more than £50.
As for the £5 hand wash car wash set ups, each to their own but I personally wouldn’t use them. Most of these places work on the basis of getting as many cars washed in the shortest possible time, as time is money to them. So most will use quite harsh / high strength products that makes it easy to remove the dirt quickly, and these harsh products carry a high risk of also stripping any wax / sealant off after a few washes.
There’s also a risk that whatever they use to wash your car (mitt or sponge) will have been used on the previous vehicle, and if that vehicle was caked in dirt, then a grit laden mitt or sponge won’t be good for your car’s paintwork. I’ve also seen these guys rubbing hard on customers’ cars on areas of stubborn dirt or baked on insects, inflicting swirls in the process.