I'd probably go "big" dog only. Newf, St Bernard, Bernese Mountain Dog sort of thing.
You can get a Pet Sitter certificate (whatever that means) on Groupon for £19 at the moment. That gives you credibility.
I'd probably start off by giving out freebies so I could get some testimonials. I'd write specifically to Crufts champions (of any level) so you could describe youself as "as selected by Crufts Winners".
£30 per night per dog.
I reckon you're looking at an initial layout of £2000 to do it right and ongoing costs of c £300 per year for advertising and website fees. If your rig is going to last 10 years you'd need 17 "dog nights" per year to break even, 34 to make it worthwhile.