It annoys me that I keep hearing the same story from
experienced licensees who are caught out in this way.
Hindsight is a wonderful thing but the best way to avoid this problem is not to sign or verbally agree to any contract in the first week or two, however reasonable it seems. It takes time to suss out the best deal. It is tempting to treat it as just another thing to cross off the list. Heaven knows the list is long enough when you take on a new pub, but frankly, you can afford to wait before you sign up.
The fact is that no right-minded supplier will cut off your supply on day one - as the current supplier they are more likely to retain your business. You should always ask for time to consider the options and read the smallprint before you agree to anything that could be construed as a contract.
The very fact that these companies try to bamboozle you on your first day shows they need the chaos of changeover day to get you to sign up - if their deals stacked up in the cold light of day and they truly valued your custom, they wouldn't hassle you when you are under such pressure.
Sadly, the best way to approach things may be to assume everyone is out to fleece you until they prove otherwise. When companies contact you and offer a deal, ask them to send you the details of the proposal in the post. Say you would like to get a legal expert to look over them before you commit, even if you don't actually do it! Often nothing will appear because their proposals don't bear close scrutiny.
Remember too that there is no such thing as a free lunch. Agents and energy consultants may well want something in return for seemingly sourcing you the best deal - either by way of payment or contract to purchase the supply through them or their preferred supplier.