Holly- It's funny that you ask that actually, because my colleague is working with a Scottie at the moment who sounds just like yours! As far as the owner is aware, there is no real reason for hating being groomed, but she's really not a fan. Ideally, what you need is a groomer with a lot of patience, who works with him to make him realise that it's a nice place to be and that it can be a positive experience. In our opinion, consistency is key with dogs like this, which is why this particular dog is only ever groomed by my colleague, and will continue to be until we've got her to a point where she's happy. Baby steps are necessary, short positive visits close together (I.e every 2-3 weeks) where he isn't pushed beyond what he is comfortable with, and with the aim to get him comfortable with the process and the environment, rather than actually aiming to finish his groom and make him look nice. It's a slow process, and sometimes it doesn't work, but we usually get good results this way. The Scottie that my colleague is working with has been to us 6 times now, and her progress is slow, but she is progressing, which is the main thing. She now happily walks into our salon and is genuinely pleased to see us, and doesn't mind when her mum leaves. My colleague even managed to do her nails last time, which is a big result! In my opinion, there is no quick fix, so it's just a case of finding the right groomer, sticking with it, and lots of patience, praise and no pressure on the dog.
Personally, my favourite clippers are Heiniger Saphirs- they are amazing! I couldn't be without them! They are expensive, but worth every penny. With clipping, it's important to note that you'll get a better finish if you're clipping a freshly clean, dry coat, and this is much better for your blades too (apologies if you already knew that, but if not, then it might be why you're not getting quite the right finish with your Wahls) 
I did my City and Guilds at a college just outside London. I actually did a month long intensive course, and was qualified at the end of it! In all honesty, I don't necessarily think that's good, as I feel that people need a lot more practical experience before they qualify, but it's actually quite a common way to train, especially if you go into it as a second career (like I did), because apprenticeships aren't usually available to people over 25, and other than that, there aren't many hands on routes. I then went and worked for someone else to build up my skills and experience, and now have my own salon
Dog Grooming is a largely unregulated industry (quite frightening really!), and there is actually no requirement to be qualified in order to set up as a dog groomer. They are looking to change this, but at the moment that's how it is. Most people wouldn't even think to check if their groomer is qualified, because you would quite rightly assume that they would be! Sorry, that was a bit of a tangent there! It's a really great job, and I absolutely love it, so I say go for it
I spent years in a 'professional' job in a stressful industry, which I think people thought I was mad to give up, but I've genuinely never been happier in my work!