There are statutory rights that you have anyway, such as for a full time position you are entitled to 28 days leave, so the contract normally covers extra stuff (so I have 33 days under my current one).

Plus it covers things like disciplinary procedures, working hours etc. It's really weird to me that you don't have that. Are you sure it's normal?

To address your specific point, you can mostly be fired without reason if you're a new employee. You get more rights after 2 years so companies generally have a procedure to go through after that. You can always appeal to an employment tribunal but they won't take much notice if you've been there a couple of months and got fired for not doing your job.