This is in the U.K. Make sure you get the other offer. You don’t want to mess the first offer up and then have the other offer fall through. As soon as you actually have the other job, you call the first company immediately and tell them that you changed your mind and what you need to do to cancel your contract.
They will be annoyed. Very annoyed. But they are unlike to cause you trouble (German companies are likely to cause you trouble, just out of principle, but you’re in the U.K. ). They may threaten you, but are unlikely to follow through with it. Any agency you used will also be pissed off with you in a major way. So don’t do this again.