I've been a professional house/pet sitter on and off for years now but I don't really do it in the UK.
For me it's a dream job, even if there are some nutty owners out there.
It took a while to build up the right experience and references but it was well worth it, now I can earn a very good wage and be extremely choosy about the sits I pick.
I started out using social media and house sitting sites. Now I mostly get requests through personal recommendations and word of mouth and, of course, I have my regulars.
Things that will set you apart from the crowd -
Handymen and keen gardeners are often needed.
Ample experience with animals, especially farm animals or exotics.
Being extremely flexible with dates - it's not uncommon for owners to want to extend.
Leaving the place spotlessly clean, with a basic food shop and home cooked meal ready for them on their return.
Offering lunch or coffee in a neutral location for the first meet, keep in mind you're interviewing them as much as they're interviewing you.
Experience looking after pools, spa pools and hot tubs (if you're looking at those kinds of properties, I don't accept any sit that doesn't have a pool now)
Contactable references, an up to date DBS check, animal first aid certificates...