Insurance company cancelling a policy

Hello, new member here!
Today we received a letter from the insurance brokers who arranged insurance for our van
Having moved we informed them of new address, we received the letter today sayong the company they sourced the insurance from where not prepared to continue to cover the vehicle as postcode was now unsuitable (surprising as the new address is a prestige complex of apartments with gated entrance, 24/7 concierge and CCTV all over the site!) they say this is an enforced cancellation???
They are also saying that they are going to chatge us £70 to cancel the policy!!!
Any advice on how to respond, what to write or speak to about this (mainly the cancellation charge!) would be appreciated
Thank you...
Woukd attach the letter body but cannot work out how
Autonetinsurance co uk is the broker, who have been extremely vague and unhelpful...