When you purchase a flat, your conveyancer will get information about the property from the Land Registry. This will include the title deeds and other documents that report on whether you can sublet the property, carry out alterations, or keep pets.
Your conveyancer will review this information and report back to you on whether to go ahead and commit to purchasing the property.
If you want to view the title information in advance, you can ask your conveyancer to send it to you. Alternatively you can order a copy of the title information directly from Land Registry, but you will need to pay a fee for this.