United Kingdom has 27 festivals in this guide, spread across 22 cities. They run in May, June, July and August, and festival-day temperatures range from 18°C (64°F) to 23°C (73°F), with nights down to 10°C (50°F). That range is the reason there is no single "United Kingdom festival outfit". The answer changes by month and by site.
13 carry a high rain risk, which makes waterproof footwear a requirement rather than a precaution.
The ground matters as much as the sky. You will find open field, city park, industrial site, forest, city streets, mountain and beach, which puts waterproof boots at the top of the list for most of these events. 18 of the 27 have camping on site, so outfits have to survive living in a bag on the ground for several days.
Stylistically the crowd here leans camping, costume, boho and indie. Notting Hill Carnival is the largest of them at around 1,000,000 people, and it sets the tone the smaller ones react to. None of that is a dress code. It is what you will see, which is worth knowing if you would rather blend in or deliberately not.