I investigated this once (while I was sitting around doing not much during a 3 month notice period, funnily enough). It's as you say, the breach of a contract is something where the employer would have to sue you for damages, and if you're sitting around not doing much, it's going to be hard to argue that losing you has caused them harm, much less enough to make it worth suing over.

I didn't break the contract anyway out of some combination of feeling like it was breaking a promise and the worry that Cambridge is a small town.