OK, just pulled my tenancy agreement out and the relevant section in mine says....
Early Termination, the Tenant agrees, in the event of this Agreement being terminated before the end of the tenancy, by the tenant in any way, to pay £525 plus VAT, an agent administration fee for the entire cost of remarketing and re-letting the property; and to pay the entire costs of the check in; and any loss of rent incurred by the landlord as a result of the Tenant breach.
I believe mine is a fairly standard agreement as I have a sample copy of a new tenancy I'm looking at to hand too and that says pretty much exactly the same thing.