In the UK, most tenancies are created when both parties to the agreement - the landlord and the tenant - sign a tenancy agreement before the start of the tenancy.
However, there are some instances where a tenancy agreement does not exist, for various reasons. We recently helped a customer who relocated to the UK and found a property to rent from a private landlord who had chosen not to use the services of a letting agent in order to save a few quid. We wold usually only recommend taking a property through a letting agent but due to a dire lack of availability in the area, the customer had little choice.
Unfortunately, the landlord knew even less about property letting and tenancy than we know about Vladimir Putin's financial affairs, so in order to help our customer move his family into the property as soon as possible with the minimum of fuss, we guided the landlord through the process, including providing and drawing up a fair and balanced tenancy agreement and advising how to register with one of the tenancy deposit schemes.
All was good until it came to signing the tenancy agreement. Despite numerous requests, the landlord has to date still not provided a signed tenancy agreement to the tenant. Now, although this is not ideal, it is not the end of the world - when the employee moved into the property and paid rent, a tenancy was created under s54(2) of the Law of Property Act 1925 which still imposes upon the landlord the repair obligations as set out in section 11 of the Landlord and Tenant Act 1985.
Naturally, the tenant is under certain obligations, including acting in a "tenant like manner" - which essentially means that the tenant must occupy the property as it were his or her own home and, for example, change lightbulbs, repressurise the boiler, unblock sinks, cut the grass, etc., etc., etc. - but the greatest drawback actually affects the landlord. In order for a landlord to make deductions from the security deposit, the details of this must be laid out in the tenancy agreement - otherwise, the landlord is not entitled to make any deduction whatsoever.
So whilst not having a tenancy agreement is not the preferred outcome, both parties do still have rights and obligations and ultimately it is the landlord who is at the greater disadvantage in such a situation.
Our advice, if you are negotiating your own tenancy, is to document all communication and save all emails so that in the rare event you are not provided with a tenancy agreement you can at least provide evidence of what was agreed in the event of a dispute.