+1 what peteb has posted.
Given the current state of the property market I doubt that an application to increase your mortgage will be entertained.
If you retain a competent building professional to draw up plans and provide specifications for the work and tie them into a package for works to improve the insulation of the house, and present all this information to the bank as the reason for seeking finance, you may have some chance of getting a personal loan to to carry out the work.
ONQ.
All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.