It literally says you're not allowed to change utility provider if you do I will charge £100 when you leave.
Ofgem have made it quite clear that a tenant who directly pays the bill has the right under consumer laws to choose their own supplier. Things are more complicated if there are a "default supplier" which the landlord have indicated in advance. That's a specific tie-in the landlord have with the supplier, not just because they don't want you to switch. You can also be required to return to original condition at end of tenancy.
Without the exact wording, and only going back your description above, I would say that there was an absolute prohibition without a qualifying default supplier clause, and it's an absolute prohibition rather than you must switch back at the end of tenancy. While a qualifying prohibition may be legal in which case there are requirement for the landlord not to without consent unreasonablely, an absolute prohibition is clearly not legal. An absolute prohibition clause should be void as a whole, in which case the £100 penalty would be void as well.
Give CAB and/or Shelter a call and tell them the exact wordings of the clause and see what they think.