To put the description of your insurance affairs another way:
You were quoted an insurance premium of around �1500 which you elected to pay in monthly installments. You have paid 3(?) months, therefore you have only given the insurer �375 of the �1500 you agreed to pay. By terminating the contract early they could demand the full remainder (�1125) be paid to them but instead the contract includes a short-term premium rate to be applied for early cancellation. Their calculation as to the shortfall of the premium is �500, a figure that will be similar to many other UK insurers in the same circumstances. If you refuse to pay the debt the insurer can seek enforcement through the courts.
Here is an explanation of policy cancellation as provided by
Budget, a sister company of
ibuyeco (i.e. both are brands within
BGL Group)
http://www.budgetinsurance.com/Car-Insurance/C ancellation-of-Car-Insurance-Policies/
and this gives an indication of typical short-term premium rates
http://www.yesinsurance.co.uk/car-insurance/po licy-insurance-cancellation.html