I can see what you're trying to do, and some customers may like this. However, I would say in good larger kennels, staff members will work with the same group of dogs each day, so they will still get to know them and their needs. I worked in a kennel with a capacity for over 30 dogs and about 10 cats- we definitely knew all the animals by name, not number, and they all got individual attention several times a day. Customers were obviously happy as there were lots of repeat customers.
Although the Kennel had this capacity, it was only ever full in school holidays, bank holiday weekends etc. There were much quieter times, when the kennels were less than 50% full. Whatever you business plan is, I think you have to plan for periods when you are not full to capacity- at least for the first year. At busy times, you will be turning lots of people away, and losing them as potential future customers. I would also say if you turn a regular customer away, you risk losing their business permanently as well.
Our policy was always that dogs would be walked on lead and have free play several times a day in a fenced area, so I think on lead walks only are fine/standard!
If you can truly afford to go in at £15 a day, then I would go in at this level, with a view to increasing your prices a small amount in 18 months when you have built up some regular customers etc.
Good luck, whatever you decide.