Premier credit cards are only available to individuals who meet very strict eligibility criteria. In practical terms this means you'll either have to have a high net worth or a high salary. The qualification specifics are slightly different for each product, but typically products are looking for individuals who are paid considerably more than the UK average, and some are looking for over £50,000 per ammum (there are other cards with even tougher criteria, but many of these tend to be on an "invitation only" basis). You'll also need to have an excellent score to get a premier credit card. So that means no missed payments, no spending beyond your credit limit, and a reasonable amount spent regularly on your previous cards (to demonstrate you are used to using credit and can manage it well). Most premier credit card issuers will also be looking for customers who are slightly older than the generic ’18 years’ required by the mass market. Indeed, many set a minimum age of 21. In practical terms, because you need to have demonstrated an excellent credit history, these age restrictions don't make the cards any more difficult to obtain.
Each premier credit card offers slightly different features. Some premier credit cards offer airmiles (with earn rates at higher levels than the standard equivalent product). Some offer concierge services (which arrange whatever you might need upon instruction), such as theatre tickets, rental cars, golf course availability, help finding the perfect gift, and much more. Some even offer purchase protection insurance and best price protection. These differences are partly for marketing reasons, to appeal to different customers, and partly due to the different arrangements the credit card platforms have. Mastercard supports different benefits to American Express, and Visa, and vice versa.
If you don't think you'll meet the required criteria to get a premier credit card, don't worry, there are still many great products available. Many platinum credit cards offer market leading rates on balance transfers or cashback. You can even get great airline rewards with many airmile credit cards.