HMRC are happy with electronic images of invoices etc. as long as they're in a WORM format (write once ready many) using a popular industry standard format, such as a pdf and properly filed/organised.
Most people fall foul because they just haphazardly put each "document" into random places on their computer using different types of file, i.e. some in word, some in excel, some as jpegs some as pdfs, and worse still, using random descriptions. Makes it incredibly hard for someone else to try to find what they're looking for, hence why some tax inspectors are unhappy and insist that you print them off yourself for them.
Others make the basic mistake of not backing up their data, and then either change their PC or it suffers a hard drive crash, meaning everything's lost!
The other topical problem is scanning into some kind of software. Unless you also scan to your own backup, you'll lose your scans if the software firm closes down (i.e. cloud) or if you stop your subscription. If a tax enquiry starts after you've stopped using that software, you may end up having to pay to resubscribe to get access, and worse case scenario, if you've stopped paying, they may have deleted your data!
So, feel free to use digital scans, but make sure you've got them well organised, in a constant format, files/directory sensibly named, and that you back up, back up and back up again!
After all, you wouldn't just "file" your paperwork in a disorganised manner in random places around the house, or just hand them all over to someone you don't know - that's a recipe for disaster, and exactly what you do with computer versions if you don't keep control and organise it properly!