Welcome to the Forum, Veronica
Do check out the Puppy Care and Breeding sections on the Front Page list of the Forum which has a number of topics that will be helpful in general regarding looking for a puppy and the Kennel Club has plenty of useful information for puppy seekers too:
https://www.thekennelclub.org.uk/getting-a-dog/
If you check the Assured Breeder lists on the Kennel Club website it will give you the contact details of ABS members who will have been vetted by KC assessors and who must abide by high welfare standards and health screening requirements in order to be listed on the site:
https://www.thekennelclub.org.uk/search ... d-breeder/
Although you have specified ‘white’ miniature schnauzers, these are a rare colour as compared to the most common pepper and salt colouring and even the less common black and black & silver. Until not many years ago, white was not a recognised colour for miniature schnauzers in the U.K. so there are far fewer reputable breeders of this colour and, if you are set on only this colour, it would mean you may need to wait longer and travel further for a responsible breeder.
Miniature schnauzers are lovely companion dogs but they are high maintenance as regards grooming and are usually quite vocal - mini schnauzers are well known for their barking - not aggressively but they will warn of any and every potential threat to their territory so training and plenty of brain challenging activities are a must.
Priorities with getting any puppy should be first and foremost the quality of the breeder as regards health testing, raising puppies in an enriched and loving environment and one that is experienced with the breed and will provide lifetime support and advice to their puppy families. Secondly is choosing a breed that suits your lifestyle and home environment. The thing to bear in mind too is that a dog can live well into their teens so you are looking 14-15 years ahead when planning to add a dog to the family and whether things are going to be similar over these years as regards your being ‘home every day’. Companion dogs do need someone around much of the time and if there is a likelihood of you not being at home for long periods every day in the future, you may need to also consider whether you can afford dog-sitters / doggy day care etc. when you are not there, It is not always possible to predict what will happen years ahead but you do need to look further than the current situation to have plans in place for your canine family member if things do change and this is another reason to find a good quality breeder not only for their advice and support but one who will also commit to taking back the puppy/dog at any time in its life if a tragedy occurs which prevents them staying with you.
It could be well worth trying to get along to Crufts this coming year where, at Discover Dogs, you can get to meet owners and talk to them about having a mini schnauzer in the family and, of course, at the ringside there will be many breeders who you can talk to and see their dogs. Miniature Schnauzers are ‘utility’ breeds so will be shown on the Sunday 12 March but the Discover Dogs section will be open every day.:
https://www.crufts.org.uk/press-releases/crufts-2023
Below are links to a couple of the miniature schnauzer breed clubs who may be able to help you too:
http://www.theminiatureschnauzerclub.co.uk/
http://northernschnauzerclub.co.uk/
Do always check out any breeder you are considering getting a puppy from and ensure you are given all relevant health test results (eye screening; DNA test for MAC) and that you feel comfortable and confident in their integrity. Good luck in your search