I'm thinking of doing some freelance work, and I've been debating if I should register a limited company (in the United Kingdom). Is there any good reason to do this? Are there any disadvantages?
Specifically in the context of freelancing―
Should it all go titsup, you will be liable only for called-up unpaid share capital—often minimal to nothing—unless you've broken the law (or have given personal guarantees, which you should avoid doing at all costs).
If you're making over ~£40k/year, you can reduce your tax burden by incorporating (and then taking salary up to the basic rate tax threshold with the remainder received by way of dividend—although note that your personal creditors may ignore dividend income when deciding your capacity for borrowing). However, beware of falling foul of IR35: if HMRC deem that you are in fact employed by your clients and not a contractor, the tax burden through using such a company will be greater than if you were paid PAYE.
Since the company will be employing you, it must have employer's liability insurance and comply with employers' health and safety obligations. You also have a duty to direct the company fairly in the interests of your creditors. Failing to do so, for example by trading whilst insolvent, can give rise to personal liability or even criminal sanctions.
The company's assets belong to the company—not you. You must be careful to use them only for the company's benefit, or else there will be tax consequences.
There are additional filing and record-keeping obligations, but the burden isn't really much more cumbersome than that which you would have to do anyway for an unincorporated business. Biggest things are minutes, payroll, annual report/accounts and corporation tax returns.
When you've finished trading, a company can be costly and complicated to shut down.