People do apply to other firms and then renege if they receive another offer. It isn’t a great thing to do but the firm you renege on will prefer you doing this than sitting out two years in a TC you don’t really want.
The worst that can happen is that you will be remembered by a small group of people involved in your recruitment process at your reneged firm for a period of time (few years). If you wanted to join them as a NQ some people might remember and not look upon your application favourably.
The only other risk is that someone in that small group goes to work at another firm and remembers you reneged on them elsewhere. But then that’s your reputation with one person in a firm of many people.
If you are going to do it, just make sure you do it before you receive any financial support from the firm you renege on (eg signing up to the LPC/SQE course) That’s when you are tied in more: