It was a long time ago and my memory is faulty: a small group of 4 or 5 young men and women are sent to Mars from England (perhaps to check out colonization?), using some sort of long-term human hibernation or suspended animation. They return safely, and I believe that a 2nd book has them going to the center of Earth. I don't remember much more detail except one of the male character's names might be Chris, and it wasn't a horror story (there were no aliens, monsters or mythical creatures).
I'm pretty sure you are thinking of the Chris Godfrey of UNEXA series of childrens sci-fi books by Hugh Walters. These follow the adventures of Chris Godfrey, an English astronaut, to various places in outer space, starting with the moon and including many other planets in the solar system. He is accompanied on his journeys by a team of astronauts from other countries. There are 21 books in the series, written from 1957 to 1970.
a small group of 4 or 5 young men and women are sent to Mars from England
Most of the books follow a group of 4 (male) astronauts on various adventures. The lead astronaut, Chris Godfrey, is distinctly English- as are some other characters. In the early books they work for the British space program, although in later ones the space programs of various countries are united into a single program (UNEXA). The rest of the team is: Serge Smyslov (Russian), Morrey Kant (American) and Tony Hale (British). The early stories centre round the moon. Mars is visited in book 6, Destination Mars.
using some sort of long-term human hibernation or suspended animation
In book 10, Spaceship to Saturn, the crew are put into 'hypothermia' because of the long journey time. Later books also use the device.
I believe that a 2nd book has them going to the center of Earth
Book number 11 is The Mohole Mystery, which has the crew descending into a very deep cave, instead of exploring outer space.
one of the male character's names might be Chris
The main character is called Chris Godfrey
there were no aliens, monsters or mythical creatures
Other life forms are absent from most of the books.