The band formed in London in 1969 and was primarily the collaboration of David Byron's vocals, the guitar of Mick Box, and keyboards of Ken Hensley. Box and Byron formed an immediate song-writing partnership that drove the band's sound.The band's debut was entitled "Very 'eavy ... Very 'umble" in the UK but was self-titled in the US where audiences couldn't parse the accent and wouldn't get the Copperfield reference. They chose their name in 1969 during the centennial of Dickens' death, choosing one of Dickens' most memorable villains from David Copperfield.
Box claims to have had a passion for the music of Vanilla Fudge which was heavy on Hammond organ with guitar overlay. This sound formed the backbone of much of what Uriah Heep would record.
Main Creative Force
Box and Byron were the primary songwriting duo and probably sculpted the true sound of Uriah Heep from the beginning. But it is Mick Box who has remained with the band and continues to drive the sound. Box's guitar work probably defines the sound every bit as much as the heavy keyboards.
Most Likely To Be A Mainstream Rocker
It's very hard to pinpoint any member of the band who wasn't realistically dedicated to the overall progressive sound of Uriah Heep. But the lineup of the band has changed quite a bit over the years and a lot of artists have come and gone so I can pull a fast one here and name Nigel Olsson who played the drums at the recommendation of Elton John during their early days. Olsson later moved back to Elton's band which is clearly a more mainstream gig so I will use him as the answer on this part.
First Hint of Prog Brilliance
The band's second album "Salisbury" was distinctly progressive in nature. It included a 16 minute title track with full orchestra. Critics at the time hailed it as a heavy metal album with prog rock complexity.
The 1972 fourth album, "Demons and Wizards" was one of their most popular ever and fully developed the progressive nature of the band. It was thoroughly immersed in fantasy and medieval themes and included some of their most complex guitar - keyboard interplay. "The Wizard" and "Easy Livin'" were both hits in England while the latter was the band's only true hit in the US when it reached the top 40. While Heep has never been a band that can consistently sell out arenas, they have a strong and dedicated fan following and some of their music is very recognizable. They are a bigger hit among the metal community of Europe, particularly among Dutch and German fans.
Influenced By Them
If there is a prototypical sound for heavy metal singers, David Byron established it on early Uriah Heep records. The complex blend of organ and guitar was also incorporated by a great many other bands who would combine that with more commercial sounds. A lot of the early metal giants like Judas Priest show obvious connections to Heep's sound and you can show clear similarities in dozens of artists in the metal category.
The band continues to perform and continues to be a huge hit among European audiences, particularly in eastern Europe and the Germanic countries. The fact that there is a growing prog metal scene since the late eighties confirms that the sound Uriah Heep originated way back when was a valid sound. Today, other bands like Dream Theater have a sophistication and artistry that exceeds Heep, but the vocal style and the key ingredients are the same. The connection is pretty obvious. I would calculate their RRHOF odds as somewhere between 0% and 1%.
My Personal Take
The first album I heard by UH was "Demons and Wizards" and I was sold on their sound from the start. In general, they were heavier than I liked in my record collection, but I definitely liked most of what I heard.