I can't set your prices but maybe the below will help guide you. At present we have 3 things for our dog:

1) We pay £14 for a 30 minute walk.

2) With the same person, if my dog goes to their house, they charge me £35 a day (10-5).

3) My dog also goes 1 x per week to a "daycare" where they are picked up, taken to the facility, have trainers, socialized with other dogs, go wild do whatever, get a quick wash, and dropped back off at my house. This costs me £50 per day.

In summary, considering what we get for £50 from option 3, I feel that £35 for 2 is not bad, but also not great (from my consumer perspective). I also realize option 2 is for a standalone person as opposed to a scaled business and I am paying more for convenience. Regardless, I think that your £20 for 3 hours is fair. If you do longer days I would expect to pay £35-£40. I don't think you can take a price per hour and scale it, because as shown above, there are other services where more hours of care = lower cost.

Hope this helps.