Most cash plans are paid a month in advance, so can usually be cancelled with about 30 days' notice (though do check your policy as it can vary). This gives you the flexibility to switch plans or providers if you find you're not using the policy as expected, or if you've maxed the limits before they reset.
For example, if your policy has a max £150/year for dental and you've claimed this after six months, there's nothing stopping you from cancelling the policy and taking a new one out with a different provider, or seeing if you can upgrade to a plan with greater limits with your current insurer.
Just be careful doing this if you're waiting on a claim to be paid out and/or now have existing conditions as these are unlikely to be covered by a new policy. Plus, if the new policy has a qualifying period you would need to wait until that has passed before you can claim again.