If you're off sick with, for instance, an upset stomach and your work hear that you've been in the pub sinking pints then they'd be well within their rights to question whether you were fit to be in work.
If you're off sick with, for instance, a broken leg from a manual job then there's no way you can be in work so you can sit in the pub to your heart's content.
Ultimately it'll depend on your employers sickness and absence policy if they have one. Most policies are based on Acas guidance and will usually take a dim view of you partaking in social activities if you're claiming not to be fit for work. With some exceptions eg broken bones, stress related illnesses and others.