Daycare, boarding and kennels (DBK) are all regulated by law, walking is not.
DBK's must have a license issued by their local council. The license limits the number of dogs they're allowed at once (this number will vary depending on staffing, space etc.), and will place restrictions on how long the dogs can be left, how much exercise they need to have etc. They also specify that the owner has to have certain qualifications.
Dog walkers, on the other hand, are not regulated in any way. They don't, by law, even need to have insurance. However, good walkers will be insured and first-aid trained, and many will have qualifications in canine body language too.
However, all that said, I think the most important thing is that your dog clicks with their carer. A dog walker/daycare can be perfect on paper, but if your dog isn't comfortable then it won't work out long-term.
Ask for personal recommendations, do a free meet and greet, do some trial walks or visits and get to know the walker/carer well. You can also ask them to provide you with references from existing clients.